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Which two statements are true regarding subqueries? （）
A:A subquery can retrieve zero or more rows 
B:Only two subqueries can be placed atone level 
C:A subquery can be used only in SQL query statements 
D:A subquery can appear on either side of a comparison operator 
E:There is no limit on the number of subquery levels in the WHERE clause of a SELECT statement 

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出自：Oracle认证

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