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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 color column from VARCHAR(255) to VARCHAR(30):

ALTER TABLE cars
ALTER COLUMN year TYPE VARCHAR(4);

Result

ALTER TABLE

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);

Result

ALTER TABLE

PostgreSQL Exercises

Test Yourself With Exercises

Exercise:

Write the correct SQL statement to change the data type of the year field from INT to VARCHAR(4):

 cars
 year  VARCHAR(4);

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