这题为什么选BD啊，我看只有B是对的
这题为什么选BD啊，我看只有B是对的
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.
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.
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你能帮帮他们吗