OCP 1Z0 051 133

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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.

B 不对，同一层可用多个子旬询

SQL> SELECT *  2    FROM dept  3   WHERE EXISTS (SELECT * FROM emp WHERE emp.deptno = dept.deptno AND emp.empno = 7839)  4      OR EXISTS (SELECT * FROM emp WHERE emp.deptno = dept.deptno AND emp.empno = 7566)  5      OR EXISTS (SELECT * FROM emp WHERE emp.deptno = dept.deptno AND emp.empno = 7654);    DEPTNO DNAME          LOC---------- -------------- -------------        10 ACCOUNTING     NEW YORK        20 RESEARCH       DALLAS        30 SALES          CHICAGO3 rows selected

C 不对，子查询可用于查询、更新、删除
E 不对，见 http://docs.oracle.com/cd/E11882_01/server.112/e41084/queries007.htm#SQLRF52357
A subquery in the WHERE clause of a SELECT statement is also called a nested subquery. You can nest up to 255 levels of subqueries in the a nested subquery.

Answer: AD

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