PostgreSQL ALTER COLUMN
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 VARCHAR(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 COLUMN year TYPE VARCHAR(4);
Result
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 COLUMN color TYPE VARCHAR(30);