The ALTER TABLE Statement
To change the data type, or the size of a table column we have to use the
ALTER TABLE
statement.
The
ALTER TABLE
statement is used to add, delete,
or modify columns in an existing table.
The
ALTER TABLE
statement is also used to add and drop various constraints on an existing table.
ALTER COLUMN
We want to change the data type of the
year
column of the
cars
table from
INT
to
VARCAHR(4)
.
To modify a column, use the
ALTER COLUMN
statement and the
TYPE
keyword followed by the
new data type:
Example
Change the
year
column from
INT
to
VARCHAR(4)
:
ALTER TABLE cars
ALTER COLUMN
year TYPE VARCHAR(4);
Note:
Some data types cannot be converted if the column has value.
E.g. numbers can always be converted to text, but text cannot always be
converted to numbers.
Change Maximum Allowed Characters
We also want to change the maximum number of characters allowed in the
color
column of the
cars
table.
Use the same syntax as above, use the
ALTER COLUMN
statement and the
TYPE
keyword followed by the
new data type:
Example
Change the
color
column from
VARCHAR(255)
to
VARCHAR(30)
:
ALTER TABLE cars
ALTER COLUMN color TYPE VARCHAR(30);
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