I am reading the Postgres 7.3 documentation to add a constraint to a PostgreSQL table.
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.
I don't understand what it means by "which cannot be written as a table constraint"?
What is table constraint? What constraint doesn't check immediately?