OCP-1Z0-051-Question Analysis-28th question

28. Which two statements are true regarding constraints? (Choose two .)
A. A foreign key cannot contain NULL values.
B. A column with the UNIQUE constraint can contain NULL values.
C. A constraint is enforced only for the INSERT operation on a table.
D. A constraint can be disabled even if the constraint column contains data.
E. All constraints can be defined at the column level as well as the table level.


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 unique constraint can contain 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: the constraints on the field can be disabled (correct) even if the field contains data)
E: All constraints can be defined at the column level and table level (incorrect, Not Null can only be defined at the column level ).