SQL语言中，级联删除的短语是（　　）。

A．WHEN DELETE RESTRICT

B．ON DELETE RESTRICT

C．0N DELETE SET NULL

D．0N DELETE CASCADE

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SQL语言中，级联删除的短语是ON DELETE CASCADE。

1 已知关系R与S如下所示，关系R的主码为A；关系S的主码为D，外码为A。

A．(11，21)

B．(22，NULL)

C．(33，33)

D．(44，11)

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A．SELECT sno FROM sc WHERE grade=NULL

B．SELECT sno FROM sc WHERE grade IS

C．SELECT Sno FROM sc WHERE grade IS NULL

D．SELECT sno FROM sc WHERE grade=0

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3 检索“至少选修课程号为'C1'，的学生姓名(SNAME)”的SQL语句是（　　）。

A．SELECT sname FROM S，SC WHERE cno='C1'

B．SELECT sname FROM S，SC WHERE cno='C1'AND sno=Ssno

C．SELECT sname FROM S，SC FOR cno='C1'

D．SELECT sname FROM S，sc FOR cno='C1' AND sno=Ssno

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