（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.

答案：（A、D）

解析：

-- B（X）

SELECT a.empno, a.ename, b.job, b.sal, c. deptno

FROM (SELECT empno, ename FROM emp) a, (SELECT empno, job, sal FROM emp) b, (SELECT empno, deptno FROM emp) c

WHERE a.empno = b.empno AND a.empno = c.empno;

-- D正确

SELECT * FROM emp WHERE (SELECT sal FROM emp WHERE empno = 7369) = sal;

-- E（X），WHERE子句中的子查询的嵌套深度最大为255

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