convert( 表达式,数据类型 )
cast( 表达式 as 数据类型 )
可转换的类型有 字符串char、日期date、时间time、日期时间datetime、浮点型decimal、整数signed、无符号整数unsigned。
mysql> select cast(now() as signed);
+-----------------------+
| cast(now() as signed) |
+-----------------------+
| 20201030042922 |
+-----------------------+
1 row in set (0.00 sec)
mysql> select convert(now(),signed);
+-----------------------+
| convert(now(),signed) |
+-----------------------+
| 20201030042933 |
+-----------------------+
1 row in set (0.00 sec)
mysql> select cast(now() as char);
+---------------------+
| cast(now() as char) |
+---------------------+
| 2020-10-30 04:29:44 |
+---------------------+
2、使用date_format()函数实现日期格式的转换
date_format()函数可以以不同的格式显示日期/时间数据，可以实现日期转换成字符串。
语法格式：
date_format(date,format)
date为合法的日期，format为规定日期/时间的输出格式。
可以使用的格式有：
mysql> select date_format(now(),'%Y-%m-%d');
+-------------------------------+
| date_format(now(),'%Y-%m-%d') |
+-------------------------------+
| 2020-10-30 |
+-------------------------------+
示例2：从学生表 ‘student’ 中查询前2人现在的年龄。
mysql> select * from student limit 2;
+------------+-----------+-----+------------+-----------------------+---------+
| Sid | Sname | Sex | Brithdate | Specialty | AScores |
+------------+-----------+-----+------------+-----------------------+---------+
| 2011216001 | 赵成刚 | 男 | 1992-05-05 | 计算机应用技术 | 405.0 |
| 2011216002 | 李婧 | 女 | 1992-01-06 | 计算机应用技术 | 395.5 |
+------------+-----------+-----+------------+-----------------------+---------+
mysql> select sname,
-> convert(date_format(now(),'%Y'),signed)-convert(date_format(Brithdate,'%Y'),signed) as '年龄'
-> from student
-> limit 2;
+-----------+--------+
| sname | 年龄 |
+-----------+--------+
| 赵成刚 | 28 |
| 李婧 | 28 |
+-----------+--------+
3、使用str_to_date()函数实现字符串类型转换日期类型
str_to_date()函数可以将时间格式的字符串按照所指定的显示格式（format）转换为不同的时间类型。
语法格式：
str_to_date(date,format)
mysql> select str_to_date('2020-10-3','%Y-%m-%d');
+-------------------------------------+
| str_to_date('2020-10-3','%Y-%m-%d') |
+-------------------------------------+
| 2020-10-03 |
+-------------------------------------+
mysql> select str_to_date('2020-10-3','%Y');
+-------------------------------+
| str_to_date('2020-10-3','%Y') |
+-------------------------------+
| 2020-00-00 |
+-------------------------------+
mysql> select str_to_date('10:11','%H:%i:%S');
+---------------------------------+
| str_to_date('10:11','%H:%i:%S') |
+---------------------------------+
| 10:11:00 |
+---------------------------------+
