单项选择题
以下（）选项在设置时要保证从表中的外键字段允许为空。
A.ON DELETE RESTRICT
B.ON DELETE CASCADE
C.ON DELETE SET NULL
D.以上答案都不正确
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单项选择题
A.ON DELETE RESTRICT
B.ON DELETE CASCADE
C.ON DELETE SET NULL
D.以上答案都不正确
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