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3.5.3 Adding a Constraint

To add a constraint, the table constraint syntax is used. For example:

ALTER TABLE products ADD CHECK (name <> ”);
ALTER TABLE products ADD CONSTRAINT some_name UNIQUE 
  (product_no);
ALTER TABLE products ADD FOREIGN KEY (product_group_id) 
  REFERENCES product_groups;

To add a not-null constraint, which cannot be written as a table constraint, use this syntax:

ALTER TABLE products ALTER COLUMN product_no SET NOT NULL;

The constraint will be checked immediately, so the table data must satisfy the constraint before it can be added.