This part describes the use of the SQL language in PostgreSQL. We start with describing the general syntax of SQL, then explain how to create the structures to hold data, how to populate the database, and how to query it. The middle part lists the available data types and functions for use in SQL commands. The rest treats several aspects that are important for tuning a database for optimal performance.
The information in this part is arranged so that a novice user can follow it start to end to gain a full understanding of the topics without having to refer forward too many times. The chapters are intended to be self-contained, so that advanced users can read the chapters individually as they choose. The information in this part is presented in a narrative fashion in topical units. Readers looking for a complete description of a particular command should see Part VI.
这一部分的信息是这样安排的：新手可以从头读到尾， 便可以获取有关主题的完整了解，而不需要向前
引用太多的次数。 里面的章节是设计成自包含的，这样高级用户就可以选择独立的章节来阅读。 这部分
的信息是按照主题单元以叙述的方式组织的。 如果你需要了解特定命令的完整描述，那么应该看看
Part VI
。
Readers of this part should know how to connect to a PostgreSQL database and issue SQL commands. Readers that are unfamiliar with these issues are encouraged to read Part I first. SQL commands are typically entered using the PostgreSQL interactive terminal psql, but other programs that have similar functionality can be used as well.
本书的读者应该知道如何与一个 PostgreSQL 数据库连接并发出 SQL 命令。 我们建议那些不熟悉
这些方面的读者首先阅读
Part I
。 通常 SQL 命令是用 PostgreSQL 交互终端 psql 输入的，但其
它有类似功能的程序也可以使用。
SQL语法（SQL Syntax)
语法结构(Lexical Structure)
SQL input consists of a sequence of commands. A command is composed of a sequence of tokens, terminated by a semicolon (";"). The end of the input stream also terminates a command. Which tokens are valid depends on the syntax of the particular command.
A token can be a key word, an identifier, a quoted identifier, a literal (or constant), or a special character symbol. Tokens are normally separated by whitespace (space, tab, newline), but need not be if there is no ambiguity (which is generally only the case if a special character is adjacent to some other token type).
For example, the following is (syntactically) valid SQL input:
比如，下列命令是（语法上）合法的 SQL 输入：
SELECT * FROM MY_TABLE;
UPDATE MY_TABLE SET A = 5;
INSERT INTO MY_TABLE VALUES (3, 'hi there');
This is a sequence of three commands, one per line (although this is not required; more than one command can be on a line, and commands can usefully be split across lines).
Additionally, comments can occur in SQL input. They are not tokens, they are effectively equivalent to whitespace.
另外，在 SQL 输入里可以有注释。 它们不是记号，它们实际上等效于空白。
The SQL syntax is not very consistent regarding what tokens identify commands and which are operands or parameters. The first few tokens are generally the command name, so in the above example we would usually speak of a "SELECT", an "UPDATE", and an "INSERT" command. But for instance the UPDATE command always requires a SET token to appear in a certain position, and this particular variation of INSERT also requires a VALUES in order to be complete. The precise syntax rules for each command are described in Part VI.
如果从哪些记号标识命令，哪些是操作数或参数的角度考虑， SQL 语法并不是非常一致。通常头几个
记号是命令名字， 因此上面的例子我们通常可以说是一个"SELECT"， 一个"UPDATE"，和一
个"INSERT"命令。 不过， UPDATE 命令总是要求一个 SET 在某个位置出现，并且这个变体的
INSERT 还要求有一个 VALUES 才完整。每条命令的准确语法规则都在 Part VI 里描写。
标识和关键字 (Identifiers and Key Words)
Tokens such as SELECT, UPDATE, or VALUES in the example above are examples of key words, that is, words that have a fixed meaning in the SQL language. The tokens MY_TABLE and A are examples of identifiers. They identify names of tables, columns, or other database objects, depending on the command they are used in. Therefore they are sometimes simply called "names". Key words and identifiers have the same lexical structure, meaning that one cannot know whether a token is an identifier or a key word without knowing the language. A complete list of key words can be found in Appendix C.
SQL identifiers and key words must begin with a letter (a-z, but also letters with diacritical marks and non-Latin letters) or an underscore (_). Subsequent characters in an identifier or key word can be letters, underscores, digits (0-9), or dollar signs ($). Note that dollar signs are not allowed in identifiers according to the letter of the SQL standard, so their use might render applications less portable. The SQL standard will not define a key word that contains digits or starts or ends with an underscore, so identifiers of this form are safe against possible conflict with future extensions of the standard.
The system uses no more than NAMEDATALEN-1 bytes of an identifier; longer names can be written in commands, but they will be truncated. By default, NAMEDATALEN is 64 so the maximum identifier length is 63 bytes. If this limit is problematic, it can be raised by changing the NAMEDATALEN constant in src/include/pg_config_manual.h.
Key words and unquoted identifiers are case insensitive. Therefore:
标识符和关键字名字都是大小写无关的。因此
UPDATE MY_TABLE SET A = 5;
can equivalently be written as:
也可以等效地写成
uPDaTE my_TabLE SeT a = 5;
A convention often used is to write key words in upper case and names in lower case, e.g.:
一种好习惯是把关键字写成大写，而名字等用小写。
UPDATE my_table SET a = 5;
There is a second kind of identifier: the delimited identifier or quoted identifier. It is formed by enclosing an arbitrary sequence of characters in double-quotes ("). A delimited identifier is always an identifier, never a key word. So "select" could be used to refer to a column or table named "select", whereas an unquoted select would be taken as a key word and would therefore provoke a parse error when used where a table or column name is expected. The example can be written with quoted identifiers like this:
Quoted identifiers can contain any character, except the character with code zero. (To include a double quote, write two double quotes.) This allows constructing table or column names that would otherwise not be possible, such as ones containing spaces or ampersands. The length limitation still applies.
A variant of quoted identifiers allows including escaped Unicode characters identified by their code points. This variant starts with U& (upper or lower case U followed by ampersand) immediately before the opening double quote, without any spaces in between, for example U&"foo". (Note that this creates an ambiguity with the operator &. Use spaces around the operator to avoid this problem.) Inside the quotes, Unicode characters can be specified in escaped form by writing a backslash followed by the four-digit hexadecimal code point number or alternatively a backslash followed by a plus sign followed by a six-digit hexadecimal code point number. For example, the identifier "data" could be written as
The following less trivial example writes the Russian word "slon" (elephant) in Cyrillic letters:
下面的小例子少写俄语单词“slon“为西里尔字母(大象):
U&"\0441\043B\043E\043D"
If a different escape character than backslash is desired, it can be specified using the UESCAPE clause after the string, for example:
一个不同的转义字符比较需要反斜杠，它也可以通过UESCAPE短词放到字符串后面 ，例如：
U&"d!0061t!+000061" UESCAPE '!'
The escape character can be any single character other than a hexadecimal digit, the plus sign, a single quote, a double quote, or a whitespace character. Note that the escape character is written in single quotes, not double quotes.
转义字符可以为任意单一字符而不是一个十六进制数字，加号，一个单一引用，一个双引用或者空白字符。注意
转义字符写到单引用中，而不是双引用。
To include the escape character in the identifier literally, write it twice.
在标识符字面上要包含escape字符，重复描述它两次
The Unicode escape syntax works only when the server encoding is UTF8. When other server encodings are used, only code points in the ASCII range (up to \007F) can be specified. Both the 4-digit and the 6-digit form can be used to specify UTF-16 surrogate pairs to compose characters with code points larger than U+FFFF, although the availability of the 6-digit form technically makes this unnecessary. (Surrogate pairs are not stored directly, but combined into a single code point that is then encoded in UTF-8.)
Quoting an identifier also makes it case-sensitive, whereas unquoted names are always folded to lower case. For example, the identifiers FOO, foo, and "foo" are considered the same by PostgreSQL, but "Foo" and "FOO" are different from these three and each other. (The folding of unquoted names to lower case in PostgreSQL is incompatible with the SQL standard, which says that unquoted names should be folded to upper case. Thus, foo should be equivalent to "FOO" not "foo" according to the standard. If you want to write portable applications you are advised to always quote a particular name or never quote it.)
There are three kinds of implicitly-typed constants in PostgreSQL: strings, bit strings, and numbers. Constants can also be specified with explicit types, which can enable more accurate representation and more efficient handling by the system. These alternatives are discussed in the following subsections.
A string constant in SQL is an arbitrary sequence of characters bounded by single quotes ('), for example 'This is a string'. To include a single-quote character within a string constant, write two adjacent single quotes, e.g., 'Diannes horse'. Note that this is not the same as a double-quote character (").
SQL 里的一个字串文本是用单引号（'）包围的任意字符序列， 比如，'This is a string'。
这种声明字串常量的方法是 SQL 标准定义的。 在这种类型的字串常量里嵌入单引号的标准兼容的做法
是敲入两个连续的单引号比如，'Diannes horse'。 但双引用字符(")是不同的。
Two string constants that are only separated by whitespace with at least one newline are concatenated and effectively treated as if the string had been written as one constant. For example:
PostgreSQL also accepts "escape" string constants, which are an extension to the SQL standard. An escape string constant is specified by writing the letter E (upper or lower case) just before the opening single quote, e.g., E'foo'. (When continuing an escape string constant across lines, write E only before the first opening quote.) Within an escape string, a backslash character (\) begins a C-like backslash escape sequence, in which the combination of backslash and following character(s) represent a special byte value, as shown in Table 4-1.
Any other character following a backslash is taken literally. Thus, to include a backslash character, write two backslashes (\\). Also, a single quote can be included in an escape string by writing \', in addition to the normal way of .
It is your responsibility that the byte sequences you create, especially when using the octal or hexadecimal escapes, compose valid characters in the server character set encoding. When the server encoding is UTF-8, then the Unicode escapes or the alternative Unicode escape syntax, explained in Section 4.1.2.3, should be used instead. (The alternative would be doing the UTF-8 encoding by hand and writing out the bytes, which would be very cumbersome.)
The Unicode escape syntax works fully only when the server encoding is UTF8. When other server encodings are used, only code points in the ASCII range (up to \u007F) can be specified. Both the 4-digit and the 8-digit form can be used to specify UTF-16 surrogate pairs to compose characters with code points larger than U+FFFF, although the availability of the 8-digit form technically makes this unnecessary. (When surrogate pairs are used when the server encoding is UTF8, they are first combined into a single code point that is then encoded in UTF-8.)
警告（Caution）
If the configuration parameter standard_conforming_strings is off, then PostgreSQL recognizes backslash escapes in both regular and escape string constants. However, as of PostgreSQL 9.1, the default is on, meaning that backslash escapes are recognized only in escape string constants. This behavior is more standards-compliant, but might break applications which rely on the historical behavior, where backslash escapes were always recognized. As a workaround, you can set this parameter to off, but it is better to migrate away from using backslash escapes. If you need to use a backslash escape to represent a special character, write the string constant with an E.
In addition to standard_conforming_strings, the configuration parameters escape_string_warning and backslash_quote govern treatment of backslashes in string constants.
The character with the code zero cannot be in a string constant.
0代码的字符不能在一个字符串常量中。
UNICODE中的转意字符串常量（String Constants with Unicode Escapes）
PostgreSQL also supports another type of escape syntax for strings that allows specifying arbitrary Unicode characters by code point. A Unicode escape string constant starts with U& (upper or lower case letter U followed by ampersand) immediately before the opening quote, without any spaces in between, for example U&'foo'. (Note that this creates an ambiguity with the operator &. Use spaces around the operator to avoid this problem.) Inside the quotes, Unicode characters can be specified in escaped form by writing a backslash followed by the four-digit hexadecimal code point number or alternatively a backslash followed by a plus sign followed by a six-digit hexadecimal code point number. For example, the string 'data' could be written as
U&'d\0061t\+000061'
The following less trivial example writes the Russian word "slon" (elephant) in Cyrillic letters:
U&'\0441\043B\043E\043D'
If a different escape character than backslash is desired, it can be specified using the UESCAPE clause after the string, for example:
U&'d!0061t!+000061' UESCAPE '!'
The escape character can be any single character other than a hexadecimal digit, the plus sign, a single quote, a double quote, or a whitespace character.
The Unicode escape syntax works only when the server encoding is UTF8. When other server encodings are used, only code points in the ASCII range (up to \007F) can be specified. Both the 4-digit and the 6-digit form can be used to specify UTF-16 surrogate pairs to compose characters with code points larger than U+FFFF, although the availability of the 6-digit form technically makes this unnecessary. (When surrogate pairs are used when the server encoding is UTF8, they are first combined into a single code point that is then encoded in UTF-8.)
Also, the Unicode escape syntax for string constants only works when the configuration parameter standard_conforming_strings is turned on. This is because otherwise this syntax could confuse clients that parse the SQL statements to the point that it could lead to SQL injections and similar security issues. If the parameter is set to off, this syntax will be rejected with an error message.
To include the escape character in the string literally, write it twice.
While the standard syntax for specifying string constants is usually convenient, it can be difficult to understand when the desired string contains many single quotes or backslashes, since each of those must be doubled. To allow more readable queries in such situations, PostgreSQL provides another way, called "dollar quoting", to write string constants. A dollar-quoted string constant consists of a dollar sign ($), an optional "tag" of zero or more characters, another dollar sign, an arbitrary sequence of characters that makes up the string content, a dollar sign, the same tag that began this dollar quote, and a dollar sign. For example, here are two different ways to specify the string "Dianne's horse" using dollar quoting:
Notice that inside the dollar-quoted string, single quotes can be used without needing to be escaped. Indeed, no characters inside a dollar-quoted string are ever escaped: the string content is always written literally. Backslashes are not special, and neither are dollar signs, unless they are part of a sequence matching the opening tag.
It is possible to nest dollar-quoted string constants by choosing different tags at each nesting level. This is most commonly used in writing function definitions. For example:
$function$
BEGIN
RETURN ($1 ~ $q$[\t\r\n\v\\]$q$);
$function$
Here, the sequence $q$[\t\r\n\v\\]$q$ represents a dollar-quoted literal string [\t\r\n\v\\], which will be recognized when the function body is executed by PostgreSQL. But since the sequence does not match the outer dollar quoting delimiter $function$, it is just some more characters within the constant so far as the outer string is concerned.
The tag, if any, of a dollar-quoted string follows the same rules as an unquoted identifier, except that it cannot contain a dollar sign. Tags are case sensitive, so $tag$String content$tag$ is correct, but $TAG$String content$tag$ is not.
如果有标签的话，一个美元符包围的字串遵循和无引号包围的标识符相同的规则， 只是它不能包含美元
符。标签是大小写相关的，因此 $tag$String content$tag$ 是正确的，而 $TAG$String
content$tag$ 不对。
A dollar-quoted string that follows a keyword or identifier must be separated from it by whitespace; otherwise the dollar quoting delimiter would be taken as part of the preceding identifier.
Dollar quoting is not part of the SQL standard, but it is often a more convenient way to write complicated string literals than the standard-compliant single quote syntax. It is particularly useful when representing string constants inside other constants, as is often needed in procedural function definitions. With single-quote syntax, each backslash in the above example would have to be written as four backslashes, which would be reduced to two backslashes in parsing the original string constant, and then to one when the inner string constant is re-parsed during function execution.
Bit-string constants look like regular string constants with a B (upper or lower case) immediately before the opening quote (no intervening whitespace), e.g., B'1001'. The only characters allowed within bit-string constants are 0 and 1.
位串常量看起来很象在开引号前面有一个 B （大写或小写）的普通字符串（它们之间没有空白）， 比如
B'1001'。位串常量里可以用的字符只有 0 和 1。
Alternatively, bit-string constants can be specified in hexadecimal notation, using a leading X (upper or lower case), e.g., X'1FF'. This notation is equivalent to a bit-string constant with four binary digits for each hexadecimal digit.
另外，位串常量可以用十六进制表示法声明，方法是使用前缀的 X （大写或者小写），比如，
X'1FF'。 这种表示法等效于一个每个十六进制位四个二进制位的位串常量。
Both forms of bit-string constant can be continued across lines in the same way as regular string constants. Dollar quoting cannot be used in a bit-string constant.
两种形式的位串常量都可以象普通字串常量那样跨行连续。 美元符包围不能用于位串常量。
数值常量（Numeric Constants）
Numeric constants are accepted in these general forms:
where digits is one or more decimal digits (0 through 9). At least one digit must be before or after the decimal point, if one is used. At least one digit must follow the exponent marker (e), if one is present. There cannot be any spaces or other characters embedded in the constant. Note that any leading plus or minus sign is not actually considered part of the constant; it is an operator applied to the constant.
These are some examples of valid numeric constants:
这里是一些合法的数值常量的例子：
1.925e-3
A numeric constant that contains neither a decimal point nor an exponent is initially presumed to be type integer if its value fits in type integer (32 bits); otherwise it is presumed to be type bigint if its value fits in type bigint (64 bits); otherwise it is taken to be type numeric. Constants that contain decimal points and/or exponents are always initially presumed to be type numeric.
The initially assigned data type of a numeric constant is just a starting point for the type resolution algorithms. In most cases the constant will be automatically coerced to the most appropriate type depending on context. When necessary, you can force a numeric value to be interpreted as a specific data type by casting it. For example, you can force a numeric value to be treated as type real (float4) by writing:
REAL '1.23' -- string style
1.23::REAL -- PostgreSQL (historical) style
These are actually just special cases of the general casting notations discussed next.
这些实际上只是下面讨论的通用转换的特例。
其他类型常量(Constants of Other Types)
A constant of an arbitrary type can be entered using any one of the following notations:
任意类似的常量可以用下列表示法中的任何一种来输入：
type 'string'
'string'::type
CAST ( 'string' AS type )
The string constant's text is passed to the input conversion routine for the type called type. The result is a constant of the indicated type. The explicit type cast can be omitted if there is no ambiguity as to the type the constant must be (for example, when it is assigned directly to a table column), in which case it is automatically coerced.
在字串常量的文本将传递给那种叫 type 的类型的输入转换过程。 结果是这种类型的一个常量。如果不
存在该常量所属类型的歧义， 那么明确的类型映射可以省略（比如，当你把它直接赋予一个表字段的时
候）， 这种情况下它会自动转换。
The string constant can be written using either regular SQL notation or dollar-quoting.
字串常量可以用普通 SQL 表示法或者美元符包围来书写。
It is also possible to specify a type coercion using a function-like syntax:
我们还可以用函数样的语法来声明类型转换：
typename ( 'string' )
but not all type names can be used in this way; see Section 4.2.9 for details.
不过并非所有类型名可以这样使用；参阅 Section 4.2.9 获取细节。
The ::, CAST(), and function-call syntaxes can also be used to specify run-time type conversions of arbitrary expressions, as discussed in Section 4.2.9. To avoid syntactic ambiguity, the type 'string' syntax can only be used to specify the type of a simple literal constant. Another restriction on the type 'string' syntax is that it does not work for array types; use :: or CAST() to specify the type of an array constant.
这个::，CAST()，和函数调用语法也可以用于声明任意表达式的运行时类型转换， 如 Section 4.2.9
中讨论的那样。 但是 type 'string' 的形式只能用于声明一个文本常量的类型。 type 'string'
的另外一个限制是它不能用于数组类型；要用 :: 或者 CAST() 声明一个数组常量的类型。
The CAST() syntax conforms to SQL. The type 'string' syntax is a generalization of the standard: SQL specifies this syntax only for a few data types, but PostgreSQL allows it for all types. The syntax with :: is historical PostgreSQL usage, as is the function-call syntax.
An operator name is a sequence of up to NAMEDATALEN-1 (63 by default) characters from the following list:
一个操作符是最多 NAMEDATALEN-1 （缺省 63 个字符）个下列字符的序列：
+ - * / < > = ~ ! @ # % ^ & | ` ?
There are a few restrictions on operator names, however:
不过，对操作符名字有几个限制：
-- and /* cannot appear anywhere in an operator name, since they will be taken as the start of a comment.
• -- 和 /* 不能出现在操作符名字中的任何地方，因为它们会被当做注释开始对待。
A multiple-character operator name cannot end in + or -, unless the name also contains at least one of these characters:
~ ! @ # % ^ & | ` ?
• 多字符操作符不能以 + 或 - 结束， 除非其名字至少还包含下列操作符之一：
~ ! @ # % ^ & | ` ?
For example, @- is an allowed operator name, but *- is not. This restriction allows PostgreSQL to parse SQL-compliant queries without requiring spaces between tokens.
When working with non-SQL-standard operator names, you will usually need to separate adjacent operators with spaces to avoid ambiguity. For example, if you have defined a left unary operator named @, you cannot write X*@Y; you must write X* @Y to ensure that PostgreSQL reads it as two operator names not one.
Some characters that are not alphanumeric have a special meaning that is different from being an operator. Details on the usage can be found at the location where the respective syntax element is described. This section only exists to advise the existence and summarize the purposes of these characters.
A dollar sign ($) followed by digits is used to represent a positional parameter in the body of a function definition or a prepared statement. In other contexts the dollar sign can be part of an identifier or a dollar-quoted string constant.
Parentheses (()) have their usual meaning to group expressions and enforce precedence. In some cases parentheses are required as part of the fixed syntax of a particular SQL command.
The colon (:) is used to select "slices" from arrays. (See Section 8.14.) In certain SQL dialects (such as Embedded SQL), the colon is used to prefix variable names.
The asterisk (*) is used in some contexts to denote all the fields of a table row or composite value. It also has a special meaning when used as the argument of an aggregate function, namely that the aggregate does not require any explicit parameter.
where the comment begins with /* and extends to the matching occurrence of */. These block comments nest, as specified in the SQL standard but unlike C, so that one can comment out larger blocks of code that might contain existing block comments.
A comment is removed from the input stream before further syntax analysis and is effectively replaced by whitespace.
这里注释以 /* 开头并扩展到对应的 */。这些块注释可以嵌套，就象 SQL99 里说的那样， 但和 C
不一样，因此我们可以注释掉一大块已经包含块注释的代码。
注释在进一步的语法分析之前被从输入流删除并用空白代替。
语法的优先级（Lexical Precedence）
Table 4-2 shows the precedence and associativity of the operators in PostgreSQL. Most operators have the same precedence and are left-associative. The precedence and associativity of the operators is hard-wired into the parser. This can lead to non-intuitive behavior; for example the Boolean operators < and > have a different precedence than the Boolean operators <= and >=. Also, you will sometimes need to add parentheses when using combinations of binary and unary operators. For instance:
because the parser has no idea — until it is too late — that ! is defined as a postfix operator, not an infix one. To get the desired behavior in this case, you must write:
Table 4-2. Operator Precedence (decreasing)Operator/ElementAssociativityDescription
表/列名字分隔
PostgreSQL-风格的类型转换
数组元素的拣选
right
* / %
乘法，除法，模
IS TRUE, IS FALSE, IS UNKNOWN, IS NULL
ISNULL
检测是否为 null
NOTNULL
检测是否不为 null
(any other)
all other native and user-defined operators
BETWEEN
OVERLAPS
重叠的时间间隔
LIKE ILIKE SIMILAR
字符串模式匹配
Note that the operator precedence rules also apply to user-defined operators that have the same names as the built-in operators mentioned above. For example, if you define a "+" operator for some custom data type it will have the same precedence as the built-in "+" operator, no matter what yours does.
When a schema-qualified operator name is used in the OPERATOR syntax, as for example in:
如果在 OPERATOR 语法里使用了模式修饰的操作符名， 比如
SELECT 3 OPERATOR(pg_catalog.+) 4;
the OPERATOR construct is taken to have the default precedence shown in Table 4-2 for "any other" operator. This is true no matter which specific operator appears inside OPERATOR().
Value expressions are used in a variety of contexts, such as in the target list of the SELECT command, as new column values in INSERT or UPDATE, or in search conditions in a number of commands. The result of a value expression is sometimes called a scalar, to distinguish it from the result of a table expression (which is a table). Value expressions are therefore also called scalar expressions (or even simply expressions). The expression syntax allows the calculation of values from primitive parts using arithmetic, logical, set, and other operations.
A positional parameter reference, in the body of a function definition or prepared statement
• 一个位置参数引用，在函数声明体中。
A subscripted expression
• 一个下标表达式
A field selection expression
• 一个区域性拣选表达式
An operator invocation
• 一个操作符调用
A function call
• 一个函数调用。
An aggregate expression
• 一个标量子查询。
A window function call
•一个窗体函数的调用
A type cast
• 一个类型转换
A collation expression
• 一个集合表达式
A scalar subquery
• 一个标量子查询。
An array constructor
• 一个数组构造
A row constructor
• 一个行构造
Another value expression in parentheses (used to group subexpressions and override precedence)
另外一个在圆括弧里面的值表达式，可以用于子表达式分组和覆盖优先级。
In addition to this list, there are a number of constructs that can be classified as an expression but do not follow any general syntax rules. These generally have the semantics of a function or operator and are explained in the appropriate location in Chapter 9. An example is the IS NULL clause.
除了这个列表以外，还有许多构造可以归类为表达式，但是不遵循任何通用的语法规则。 它们通常有函
数或操作符的语义，并且在 Chapter 9 里合适的位置描述。 一个例子是 IS NULL 子句。
We have already discussed constants in Section 4.1.2. The following sections discuss the remaining options.
我们已经在 Section 4.1.2 里有讨论过的内容了。下面的节讨论剩下的选项。
列引用( Column References)
A column can be referenced in the form:
一个字段可以用下面形式的引用：
correlation.columnname
correlation is the name of a table (possibly qualified with a schema name), or an alias for a table defined by means of a FROM clause. The correlation name and separating dot can be omitted if the column name is unique across all the tables being used in the current query. (See also Chapter 7.)
A positional parameter reference is used to indicate a value that is supplied externally to an SQL statement. Parameters are used in SQL function definitions and in prepared queries. Some client libraries also support specifying data values separately from the SQL command string, in which case parameters are used to refer to the out-of-line data values. The form of a parameter reference is:
In general the array expression must be parenthesized, but the parentheses can be omitted when the expression to be subscripted is just a column reference or positional parameter. Also, multiple subscripts can be concatenated when the original array is multidimensional. For example:
The parentheses in the last example are required. See Section 8.14 for more about arrays.
最后一个例子里的圆括弧是必须的。参阅 Section 8.10 获取有关数组的更多信息。
区域性拣选（Field Selection）
If an expression yields a value of a composite type (row type), then a specific field of the row can be extracted by writing
如果一个表达式生成一个复合类型（行类型），那么用下面的方法可以抽取一个指定的字段
expression.fieldname
In general the row expression must be parenthesized, but the parentheses can be omitted when the expression to be selected from is just a table reference or positional parameter. For example:
The parentheses are required here to show that compositecol is a column name not a table name, or that mytable is a table name not a schema name in the second case.
where the operator token follows the syntax rules of Section 4.1.3, or is one of the key words AND, OR, and NOT, or is a qualified operator name in the form:
这里的 operator 记号遵循语法规则： Section 4.1.3， 或者是记号：AND， OR，和 NOT 之一。
或者是一个被修饰的操作符名
OPERATOR(schema.operatorname)
Which particular operators exist and whether they are unary or binary depends on what operators have been defined by the system or the user. Chapter 9 describes the built-in operators.
The syntax for a function call is the name of a function (possibly qualified with a schema name), followed by its argument list enclosed in parentheses:
函数调用的语法是合法函数名字（可能有模式名修饰）， 后面跟着在圆括弧里的它的参数列表：
function_name ([expression [, expression ... ]] )
For example, the following computes the square root of 2:
比如，下面的代码计算 2 的平方根：
sqrt(2)
The list of built-in functions is in Chapter 9. Other functions can be added by the user.
内置函数的列表在 Chapter 9 里。 其它函数可以由用户添加。
The arguments can optionally have names attached. See Section 4.3 for details.
参数可以附带一个可选名字请参见Section 4.3。
Note: A function that takes a single argument of composite type can optionally be called using field-selection syntax, and conversely field selection can be written in functional style. That is, the notations col(table) and table.col are interchangeable. This behavior is not SQL-standard but is provided in PostgreSQL because it allows use of functions to emulate "computed fields". For more information see Section 35.4.2.
An aggregate expression represents the application of an aggregate function across the rows selected by a query. An aggregate function reduces multiple inputs to a single output value, such as the sum or average of the inputs. The syntax of an aggregate expression is one of the following:
where aggregate_name is a previously defined aggregate (possibly qualified with a schema name), expression is any value expression that does not itself contain an aggregate expression or a window function call, and order_by_clause is a optional ORDER BY clause as described below.
The first form of aggregate expression invokes the aggregate once for each input row. The second form is the same as the first, since ALL is the default. The third form invokes the aggregate once for each distinct value of the expression (or distinct set of values, for multiple expressions) found in the input rows. The last form invokes the aggregate once for each input row; since no particular input value is specified, it is generally only useful for the count(*) aggregate function.
Most aggregate functions ignore null inputs, so that rows in which one or more of the expression(s) yield null are discarded. This can be assumed to be true, unless otherwise specified, for all built-in aggregates.
For example, count(*) yields the total number of input rows; count(f1) yields the number of input rows in which f1 is non-null, since count ignores nulls; and count(distinct f1) yields the number of distinct non-null values of f1.
比如，count(*) 生成输入行的总数； count(f1) 生成 f1 为非空的输入行数；
count(distinct f1) 生成 f1 唯一非空的行数。
Ordinarily, the input rows are fed to the aggregate function in an unspecified order. In many cases this does not matter; for example, min produces the same result no matter what order it receives the inputs in. However, some aggregate functions (such as array_agg and string_agg) produce results that depend on the ordering of the input rows. When using such an aggregate, the optional order_by_clause can be used to specify the desired ordering. The order_by_clause has the same syntax as for a query-level ORDER BY clause, as described in Section 7.5, except that its expressions are always just expressions and cannot be output-column names or numbers. For example:
SELECT array_agg(a ORDER BY b DESC) FROM table;
When dealing with multiple-argument aggregate functions, note that the ORDER BY clause goes after all the aggregate arguments. For example, write this:
SELECT string_agg(a, ',' ORDER BY a) FROM table;
not this:
SELECT string_agg(a ORDER BY a, ',') FROM table; -- incorrect
The latter is syntactically valid, but it represents a call of a single-argument aggregate function with two ORDER BY keys (the second one being rather useless since it's a constant).
If DISTINCT is specified in addition to an order_by_clause, then all the ORDER BY expressions must match regular arguments of the aggregate; that is, you cannot sort on an expression that is not included in the DISTINCT list.
Note: The ability to specify both DISTINCT and ORDER BY in an aggregate function is a PostgreSQL extension.
The predefined aggregate functions are described in Section 9.18. Other aggregate functions can be added by the user.
An aggregate expression can only appear in the result list or HAVING clause of a SELECT command. It is forbidden in other clauses, such as WHERE, because those clauses are logically evaluated before the results of aggregates are formed.
When an aggregate expression appears in a subquery (see Section 4.2.11 and Section 9.20), the aggregate is normally evaluated over the rows of the subquery. But an exception occurs if the aggregate's arguments contain only outer-level variables: the aggregate then belongs to the nearest such outer level, and is evaluated over the rows of that query. The aggregate expression as a whole is then an outer reference for the subquery it appears in, and acts as a constant over any one evaluation of that subquery. The restriction about appearing only in the result list or HAVING clause applies with respect to the query level that the aggregate belongs to.
A window function call represents the application of an aggregate-like function over some portion of the rows selected by a query. Unlike regular aggregate function calls, this is not tied to grouping of the selected rows into a single output row — each row remains separate in the query output. However the window function is able to scan all the rows that would be part of the current row's group according to the grouping specification (PARTITION BY list) of the window function call. The syntax of a window function call is one of the following:
function_name ([expression [, expression ... ]]) OVER ( window_definition )
function_name ([expression [, expression ... ]]) OVER window_name
function_name ( * ) OVER ( window_definition )
function_name ( * ) OVER window_name
where window_definition has the syntax
[ existing_window_name ]
[ PARTITION BY expression [, ...] ]
[ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
[ frame_clause ]
and the optional frame_clause can be one of
[ RANGE | ROWS ] frame_start
[ RANGE | ROWS ] BETWEEN frame_start AND frame_end
where frame_start and frame_end can be one of
UNBOUNDED PRECEDING
value PRECEDING
CURRENT ROW
value FOLLOWING
UNBOUNDED FOLLOWING
Here, expression represents any value expression that does not itself contain window function calls. The PARTITION BY and ORDER BY lists have essentially the same syntax and semantics as GROUP BY and ORDER BY clauses of the whole query, except that their expressions are always just expressions and cannot be output-column names or numbers. window_name is a reference to a named window specification defined in the query's WINDOW clause. Named window specifications are usually referenced with just OVER window_name, but it is also possible to write a window name inside the parentheses and then optionally supply an ordering clause and/or frame clause (the referenced window must lack these clauses, if they are supplied here). This latter syntax follows the same rules as modifying an existing window name within the WINDOW clause; see the SELECT reference page for details.
The frame_clause specifies the set of rows constituting the window frame, for those window functions that act on the frame instead of the whole partition. If frame_end is omitted it defaults to CURRENT ROW. Restrictions are that frame_start cannot be UNBOUNDED FOLLOWING, frame_end cannot be UNBOUNDED PRECEDING, and the frame_end choice cannot appear earlier in the above list than the frame_start choice — for example RANGE BETWEEN CURRENT ROW AND value PRECEDING is not allowed. The default framing option is RANGE UNBOUNDED PRECEDING, which is the same as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW; it sets the frame to be all rows from the partition start up through the current row's last peer in the ORDER BY ordering (which means all rows if there is no ORDER BY). In general, UNBOUNDED PRECEDING means that the frame starts with the first row of the partition, and similarly UNBOUNDED FOLLOWING means that the frame ends with the last row of the partition (regardless of RANGE or ROWS mode). In ROWS mode, CURRENT ROW means that the frame starts or ends with the current row; but in RANGE mode it means that the frame starts or ends with the current row's first or last peer in the ORDER BY ordering. The value PRECEDING and value FOLLOWING cases are currently only allowed in ROWS mode. They indicate that the frame starts or ends with the row that many rows before or after the current row. value must be an integer expression not containing any variables, aggregate functions, or window functions. The value must not be null or negative; but it can be zero, which selects the current row itself.
The built-in window functions are described in Table 9-44. Other window functions can be added by the user. Also, any built-in or user-defined aggregate function can be used as a window function.
The syntaxes using * are used for calling parameter-less aggregate functions as window functions, for example count(*) OVER (PARTITION BY x ORDER BY y). * is customarily not used for non-aggregate window functions. Aggregate window functions, unlike normal aggregate functions, do not allow DISTINCT or ORDER BY to be used within the function argument list.
Window function calls are permitted only in the SELECT list and the ORDER BY clause of the query.
More information about window functions can be found in Section 3.5, Section 9.19, Section 7.2.4.
类型的转换（Type Casts）
A type cast specifies a conversion from one data type to another. PostgreSQL accepts two equivalent syntaxes for type casts:
The CAST syntax conforms to SQL; the syntax with :: is historical PostgreSQL usage.
CAST 语法遵循 SQL；:: 的语法是 PostgreSQL 传统用法。
When a cast is applied to a value expression of a known type, it represents a run-time type conversion. The cast will succeed only if a suitable type conversion operation has been defined. Notice that this is subtly different from the use of casts with constants, as shown in Section 4.1.2.7. A cast applied to an unadorned string literal represents the initial assignment of a type to a literal constant value, and so it will succeed for any type (if the contents of the string literal are acceptable input syntax for the data type).
An explicit type cast can usually be omitted if there is no ambiguity as to the type that a value expression must produce (for example, when it is assigned to a table column); the system will automatically apply a type cast in such cases. However, automatic casting is only done for casts that are marked "OK to apply implicitly" in the system catalogs. Other casts must be invoked with explicit casting syntax. This restriction is intended to prevent surprising conversions from being applied silently.
It is also possible to specify a type cast using a function-like syntax:
我们也可以用函数样的语法声明一个类型转换：
typename ( expression )
However, this only works for types whose names are also valid as function names. For example, double precision cannot be used this way, but the equivalent float8 can. Also, the names interval, time, and timestamp can only be used in this fashion if they are double-quoted, because of syntactic conflicts. Therefore, the use of the function-like cast syntax leads to inconsistencies and should probably be avoided.
Note: The function-like syntax is in fact just a function call. When one of the two standard cast syntaxes is used to do a run-time conversion, it will internally invoke a registered function to perform the conversion. By convention, these conversion functions have the same name as their output type, and thus the "function-like syntax" is nothing more than a direct invocation of the underlying conversion function. Obviously, this is not something that a portable application should rely on. For further details see CREATE CAST.
The COLLATE clause overrides the collation of an expression. It is appended to the expression it applies to:
expr COLLATE collation
where collation is a possibly schema-qualified identifier. The COLLATE clause binds tighter than operators; parentheses can be used when necessary.
If no collation is explicitly specified, the database system either derives a collation from the columns involved in the expression, or it defaults to the default collation of the database if no column is involved in the expression.
The two common uses of the COLLATE clause are overriding the sort order in an ORDER BY clause, for example:
SELECT a, b, c FROM tbl WHERE ... ORDER BY a COLLATE "C";
and overriding the collation of a function or operator call that has locale-sensitive results, for example:
SELECT * FROM tbl WHERE a > 'foo' COLLATE "C";
Note that in the latter case the COLLATE clause is attached to an input argument of the operator we wish to affect. It doesn't matter which argument of the operator or function call the COLLATE clause is attached to, because the collation that is applied by the operator or function is derived by considering all arguments, and an explicit COLLATE clause will override the collations of all other arguments. (Attaching non-matching COLLATE clauses to more than one argument, however, is an error. For more details see Section 22.2.) Thus, this gives the same result as the previous example:
SELECT * FROM tbl WHERE a COLLATE "C" > 'foo';
But this is an error:
SELECT * FROM tbl WHERE (a > 'foo') COLLATE "C";
because it attempts to apply a collation to the result of the > operator, which is of the non-collatable data type boolean.
标量子查询(Scalar Subqueries)
A scalar subquery is an ordinary SELECT query in parentheses that returns exactly one row with one column. (See Chapter 7 for information about writing queries.) The SELECT query is executed and the single returned value is used in the surrounding value expression. It is an error to use a query that returns more than one row or more than one column as a scalar subquery. (But if, during a particular execution, the subquery returns no rows, there is no error; the scalar result is taken to be null.) The subquery can refer to variables from the surrounding query, which will act as constants during any one evaluation of the subquery. See also Section 9.20 for other expressions involving subqueries.
For example, the following finds the largest city population in each state:
比如，下面的查询找出每个州中的最大人口数量的城市：
SELECT name, (SELECT max(pop) FROM cities WHERE cities.state = states.name)
FROM states;
数组构造(Array Constructors)
An array constructor is an expression that builds an array value using values for its member elements. A simple array constructor consists of the key word ARRAY, a left square bracket [, a list of expressions (separated by commas) for the array element values, and finally a right square bracket ]. For example:
By default, the array element type is the common type of the member expressions, determined using the same rules as for UNION or CASE constructs (see Section 10.5). You can override this by explicitly casting the array constructor to the desired type, for example:
This has the same effect as casting each expression to the array element type individually. For more on casting, see Section 4.2.9.
Multidimensional array values can be built by nesting array constructors. In the inner constructors, the key word ARRAY can be omitted. For example, these produce the same result:
Since multidimensional arrays must be rectangular, inner constructors at the same level must produce sub-arrays of identical dimensions. Any cast applied to the outer ARRAY constructor propagates automatically to all the inner constructors.
因为多维数组必须是方形，同层的内层构造器必须生成同维的子数组。
Multidimensional array constructor elements can be anything yielding an array of the proper kind, not only a sub-ARRAY construct. For example:
You can construct an empty array, but since it's impossible to have an array with no type, you must explicitly cast your empty array to the desired type. For example:
It is also possible to construct an array from the results of a subquery. In this form, the array constructor is written with the key word ARRAY followed by a parenthesized (not bracketed) subquery. For example:
SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
?column?
-------------------------------------------------------------
{2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31}
(1 row)
The subquery must return a single column. The resulting one-dimensional array will have an element for each row in the subquery result, with an element type matching that of the subquery's output column.
A row constructor is an expression that builds a row value (also called a composite value) using values for its member fields. A row constructor consists of the key word ROW, a left parenthesis, zero or more expressions (separated by commas) for the row field values, and finally a right parenthesis. For example:
The key word ROW is optional when there is more than one expression in the list.
如果在列表里有多个表达式，那么关键字 ROW 是可选的。
A row constructor can include the syntax rowvalue.*, which will be expanded to a list of the elements of the row value, just as occurs when the .* syntax is used at the top level of a SELECT list. For example, if table t has columns f1 and f2, these are the same:
缺省时，ROW 表达式创建的值是一个匿名的记录类型。如果必要，你可以把它转换成一个命名的复合类
型 — 既可以是一个表的行类型，也可以是一个用 CREATE TYPE AS 创建的复合类型。 可能会需要
一个明确的转换以避免歧义。比如：
SELECT ROW(t.*, 42) FROM t;
SELECT ROW(t.f1, t.f2, 42) FROM t;
Note: Before PostgreSQL 8.2, the .* syntax was not expanded, so that writing ROW(t.*, 42) created a two-field row whose first field was another row value. The new behavior is usually more useful. If you need the old behavior of nested row values, write the inner row value without .*, for instance ROW(t, 42).
By default, the value created by a ROW expression is of an anonymous record type. If necessary, it can be cast to a named composite type — either the row type of a table, or a composite type created with CREATE TYPE AS. An explicit cast might be needed to avoid ambiguity. For example:
CREATE TABLE mytable(f1 int, f2 float, f3 text);
CREATE FUNCTION getf1(mytable) RETURNS int AS 'SELECT $1.f1' LANGUAGE SQL;
-- No cast needed since only one getf1() exists
-- 因为只有一个 getf1() 存在，所以不需要类型转换
SELECT getf1(ROW(1,2.5,'this is a test'));
getf1
-------
(1 row)
CREATE TYPE myrowtype AS (f1 int, f2 text, f3 numeric);
CREATE FUNCTION getf1(myrowtype) RETURNS int AS 'SELECT $1.f1' LANGUAGE SQL;
-- Now we need a cast to indicate which function to call:
-- 现在我们需要类型转换以表明调用哪个函数：
SELECT getf1(ROW(1,2.5,'this is a test'));
ERROR: function getf1(record) is not unique
SELECT getf1(ROW(1,2.5,'this is a test')::mytable);
getf1
-------
(1 row)
SELECT getf1(CAST(ROW(11,'this is a test',2.5) AS myrowtype));
getf1
-------
(1 row)
Row constructors can be used to build composite values to be stored in a composite-type table column, or to be passed to a function that accepts a composite parameter. Also, it is possible to compare two row values or test a row with IS NULL or IS NOT NULL, for example:
行构造器可以用于制作存储在复合类型表字段里面的复合类型值， 或者是传递给一个接受复合类型参数
的函数。还有，我们也可以比较两个行数值或者用 IS NULL 或 IS NOT NULL 测试一个行数值，比
SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same');
SELECT ROW(table.*) IS NULL FROM table; -- detect all-null rows
For more detail see Section 9.21. Row constructors can also be used in connection with subqueries, as discussed in Section 9.20.
The order of evaluation of subexpressions is not defined. In particular, the inputs of an operator or function are not necessarily evaluated left-to-right or in any other fixed order.
Furthermore, if the result of an expression can be determined by evaluating only some parts of it, then other subexpressions might not be evaluated at all. For instance, if one wrote:
then somefunc() would (probably) not be called at all. The same would be the case if one wrote:
那么 somefunc() 就（可能）根本不会被调用。 如果我们写下面的，也可能会是这样
SELECT somefunc() OR true;
Note that this is not the same as the left-to-right "short-circuiting" of Boolean operators that is found in some programming languages.
请注意这里和某些编程语言里的从左向右"短路"是不一样的。
As a consequence, it is unwise to use functions with side effects as part of complex expressions. It is particularly dangerous to rely on side effects or evaluation order in WHERE and HAVING clauses, since those clauses are extensively reprocessed as part of developing an execution plan. Boolean expressions (AND/OR/NOT combinations) in those clauses can be reorganized in any manner allowed by the laws of Boolean algebra.
因此，拿那些有副作用的函数作为复杂表达式的一部分是不明智的选择。 在 WHERE 和 HAVING 子句
里面依赖副作用或者是计算顺序是特别危险的， 因为这些子句都是作为生成一个执行规划的一部分进行
了大量的再处理。 在这些子句里的布尔表达式（AND/OR/NOT 的组合）可以以布尔代数运算律允许的
任意方式进行识别。
When it is essential to force evaluation order, a CASE construct (see Section 9.16) can be used. For example, this is an untrustworthy way of trying to avoid division by zero in a WHERE clause:
如果强制计算顺序非常重要，那么可以使用 CASE 构造（参阅 Section 9.13）。 比如，下面是一种
视图避免在 WHERE 子句里被零除的不可信的方法：
SELECT ... WHERE x > 0 AND y/x > 1.5;
But this is safe:
但是下面这样的是安全的：
SELECT ... WHERE CASE WHEN x > 0 THEN y/x > 1.5 ELSE false END;
A CASE construct used in this fashion will defeat optimization attempts, so it should only be done when necessary. (In this particular example, it would be better to sidestep the problem by writing y > 1.5*x instead.)
用这种风格的 CASE 构造会阻止优化，因此应该只在必要的时候使用。 （在这个特殊的例子里，毫无疑
问写成 y > 1.5*x 更好。）
调用函数（Calling Functions）
PostgreSQL allows functions that have named parameters to be called using either positional or named notation. Named notation is especially useful for functions that have a large number of parameters, since it makes the associations between parameters and actual arguments more explicit and reliable. In positional notation, a function call is written with its argument values in the same order as they are defined in the function declaration. In named notation, the arguments are matched to the function parameters by name and can be written in any order.
In either notation, parameters that have default values given in the function declaration need not be written in the call at all. But this is particularly useful in named notation, since any combination of parameters can be omitted; while in positional notation parameters can only be omitted from right to left.
PostgreSQL also supports mixed notation, which combines positional and named notation. In this case, positional parameters are written first and named parameters appear after them.
The following examples will illustrate the usage of all three notations, using the following function definition:
CREATE FUNCTION concat_lower_or_upper(a text, b text, uppercase boolean DEFAULT false)
RETURNS text
SELECT CASE
WHEN $3 THEN UPPER($1 || ' ' || $2)
ELSE LOWER($1 || ' ' || $2)
LANGUAGE SQL IMMUTABLE STRICT;
Function concat_lower_or_upper has two mandatory parameters, a and b. Additionally there is one optional parameter uppercase which defaults to false. The a and b inputs will be concatenated, and forced to either upper or lower case depending on the uppercase parameter. The remaining details of this function definition are not important here (see Chapter 35 for more information).
使用位置标记（Using Positional Notation）
Positional notation is the traditional mechanism for passing arguments to functions in PostgreSQL. An example is:
SELECT concat_lower_or_upper('Hello', 'World', true);
concat_lower_or_upper
-----------------------
HELLO WORLD
(1 row)
All arguments are specified in order. The result is upper case since uppercase is specified as true. Another example is:
SELECT concat_lower_or_upper('Hello', 'World');
concat_lower_or_upper
-----------------------
hello world
(1 row)
Here, the uppercase parameter is omitted, so it receives its default value of false, resulting in lower case output. In positional notation, arguments can be omitted from right to left so long as they have defaults.
使用命名标记 (Using Named Notation)
In named notation, each argument's name is specified using := to separate it from the argument expression. For example:
SELECT concat_lower_or_upper(a := 'Hello', b := 'World');
concat_lower_or_upper
-----------------------
hello world
(1 row)
Again, the argument uppercase was omitted so it is set to false implicitly. One advantage of using named notation is that the arguments may be specified in any order, for example:
SELECT concat_lower_or_upper(a := 'Hello', b := 'World', uppercase := true);
concat_lower_or_upper
-----------------------
HELLO WORLD
(1 row)
SELECT concat_lower_or_upper(a := 'Hello', uppercase := true, b := 'World');
concat_lower_or_upper
-----------------------
HELLO WORLD
(1 row)
使用混合标记（Using Mixed Notation）
The mixed notation combines positional and named notation. However, as already mentioned, named arguments cannot precede positional arguments. For example:
In the above query, the arguments a and b are specified positionally, while uppercase is specified by name. In this example, that adds little except documentation. With a more complex function having numerous parameters that have default values, named or mixed notation can save a great deal of writing and reduce chances for error.
