��⑴�测试一个子查询的结果是否有元组，即测试其是否为空关系。使用关键词exists和not
exists，例如：
select customer-name from borrower as B
where exists (select * from depositor as D
�������where D.customer-name = B.customer-name)
��⑵�测试一个子查询的结果是否有重复元组。使用关键词unique和not unique，例如，找出在Perryridge分支机构中只有一个帐户的所有客户姓名：
select T.customer-name from depositor as T
where unique (select R.customer-name
�������from account, depositor as R
�������where R.customer-name = T.customer-name and
����������R.account-number����������=
account.account-number
��������and account.branch-name = "Perryridge")�
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