多项选择题
Which two statements are true about the primary key constraint in a table？ （）
A.It is not possible to disable the primary key constraint.
B.It is possible to have more than one primary key constraint in a single table.
C.The primary key constraint can be referred by only one foreign key constraint.
D.The primary key constraint can be imposed by combining more than one column.
E.The non-deferrable primary key constraint creates an unique index on the primary key column if it is not already indexed.
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