How to alter a column's data type in a PostgreSQL table?

Question

Entering the following command into a PostgreSQL interactive terminal results in an error:

ALTER TABLE tbl_name ALTER COLUMN col_name varchar (11);

What is the correct command to alter the data type of a column?

Answer 1

See documentation here: http://www.postgresql.org/docs/current/interactive/sql-altertable.html

ALTER TABLE tbl_name ALTER COLUMN col_name TYPE varchar (11);

Answer 2

If data already exists in the column you should do:

ALTER TABLE tbl_name ALTER COLUMN col_name TYPE integer USING col_name::integer;

As pointed out by @nobu and @jonathan-porter in the comments to @derek-kromm's answer, somewhat cryptically.

Answer 3

Cool @derek-kromm, Your answer is accepted and correct, But I am wondering if we need to alter more than the column. Here is how we can do.

ALTER TABLE tbl_name 
ALTER COLUMN col_name TYPE varchar (11), 
ALTER COLUMN col_name2 TYPE varchar (11),
ALTER COLUMN col_name3 TYPE varchar (11);

Documentation

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