PostgreSQL - How to add constraint to an existing table?

You have a table already created with lots of data in PostgreSQL, now you want to add a constraint to that table. Use the ALTER TABLE add a constraint.

Here is the generic syntax to add a constraint:

ALTER TABLE table_name
	ADD CONSTRAINT constraint_name constraint_definition;

For example, to add a UNIQUE constraint to the table customers on the column customer_id:

ALTER TABLE customers
	ADD CONSTRAINT uniquectm_const UNIQUE (customer_id);

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