133. Which two statements are true regarding subqueries? (Choose two.)
A. A subquery can retrieve zero or more rows.
B. Only two subqueries can be placed at one 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.
Answer: AD
答案解析：
A,子查询可以返回一行或者多行，也可以返回0行。
B,只能有两个子查询被放在同一层，这句话不是很懂。
C,子查询不仅可以用在查询语句，也可以用在插入和更新语句。
D,子查询可以出现在任何一方的一个比较运算符。
E,WHERE子句中的嵌套子查询最多能嵌套255层