To add a check constraint to an existing table in PostgreSQL you use the ALTER TABLE command.

Step 1: Use the ALTER TABLE command

Use this command:

ALTER TABLE table_name ADD CONSTRAINT constraint_name CHECK(what_to_check);

For example, if you have a table called "products" and you wanted to make sure that the price column cannot be negative, you could do this:

ALTER TABLE products ADD CONSTRAINT price_not_negative CHECK (price >= 0);

The constraint can have more than one check. For example, you could make sure the price was greater than zero but less than 100:

ALTER TABLE products ADD CONSTRAINT valid_price_range CHECK(price > 0 AND price < 100);