Question

Two ways to add constraints on an already made table using ALTER TABLE?

a) ADD CONSTRAINT and DROP CONSTRAINT
b) UPDATE CONSTRAINT and CREATE CONSTRAINT
c) REMOVE CONSTRAINT and CHANGE CONSTRAINT
d) MODIFY CONSTRAINT and INSERT CONSTRAINT

Answer 1

To add constraints to an already made table, you can use the ADD CONSTRAINT clause for new constraints and DROP CONSTRAINT followed by ADD CONSTRAINT for modifying existing ones, which corresponds to option (a).

Step-by-step explanation:

Two ways to add constraints on an already made table using ALTER TABLE are by using the ADD CONSTRAINT clause to add a new constraint and using the DROP CONSTRAINT followed by ADD CONSTRAINT if you need to modify an existing constraint, as there is no direct UPDATE CONSTRAINT command. The correct answer is (a) ADD CONSTRAINT and DROP CONSTRAINT.

For example, to add a non-null constraint, you could use:

ALTER TABLE table_name ADD CONSTRAINT constraint_name CHECK (column_name IS NOT NULL);

And to modify an existing constraint, you might first drop it then add a new constraint with the desired properties:

ALTER TABLE table_name DROP CONSTRAINT constraint_name; ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name);