How to update a column's type (in PostgreSQL)

So, you have a table and you need to modify a column's type. The problem arise when the column is filled and the type change is incompatible, for example, from string to integer, so how we can update the type and recompute the filled values to the new type?

Don't worry, SQL is powerful enough to let you make the change in one sentence with ALTER TABLE.

In the next example, we are supposing:

• we have a table the_table
• has a string column status with values: (happy | sad)
• we want to change the column's type from string to integer and set values as happy=1 | sad=2

Yes, I know using strings for status instead creating a table with the allowed statuses is a bad decision, but is needed for this example 😄

Easy, just run:

ALTER TABLE the_table
  ALTER COLUMN status TYPE INT
    USING (CASE WHEN status = 'happy' THEN 1 ELSE 3 END);

A note. If the column status has a default value, for example happy you first need to remove that default constraint, update the type and then set the new default value:

BEGIN;

ALTER TABLE the_table ALTER COLUMN status DROP DEFAULT;
ALTER TABLE the_table
  ALTER COLUMN status TYPE INT
    USING (CASE WHEN status = 'happy' THEN 1 ELSE 3 END);
ALTER TABLE the_table ALTER COLUMN status SET DEFAULT 1;

COMMIT;

In this case, because we are executing three sentences, we are running within a transaction to avoid undesired problems if one of the sentences fails.