29. Which two statements are true regarding constraints? (Choose two .)
A. A foreign key cannot contain NULL values.
B. The column with a UNIQUE constraint can store NULLS.
C. A constraint is enforced only for an INSERT operation on a table.
D. You can have more than one column in a table as part of a primary key.
Answer: BD
A: foreign key constraints cannot contain null values (incorrect, only non-empty constraints and primary key constraints cannot contain null values ).
B: The field with unique constraints can store NULL values (correct, because NULL <> NULL)
C: The constraints on a table only apply to the insert operation (errors, Update, and delete also apply ).
D: more than one field can be defined as the primary key in a table (correct, multiple fields can be defined as the primary key at the same time, such as CONSTRAINT it_pk primary key (ord_no, item_no ))