zoukankan      html  css  js  c++  java
  • 【2019年8月版】OCP 071认证考试原题-第42题

    Choose two

    Examine this SQL statement:

    DELETE FROM employees e

    WHERE EXISTS

    (SELECT 'dummy'

    FROM emp_history

    WHERE employee_id = e.employee_id)

    Which two are true?

    A) The subquery is executed for every row in the EMPLOYEES table.

    B) The subquery is not a correlated subquery.

    C) The subquery is executed before the DELETE statement is executed.

    D) All existing rows in the EMPLOYEEE table are deleted.

    E) The DELETE statement executes successfully even if the subquery selects multiple rows.

    Answer::AE

    (解析:这又是一个关联子查询的考题,出现过多次,A 答案大家要注意。)

    关联子查询:

    1、 先执行主查询,对于主查询返回的每一行数据,都会造成子查询执行一次

    2、 然后子查询返回的结果又传给主查询

    3、 主查询根据返回的记录做出判断

    )

    注意这道题答案 E 是对的,与前面第 14 题的答案 D 上有区别,要分别对待。

    create table emp2 as select * from emp where deptno=20;

    update emp2 set empno=7934 where ename=’SCOTT’;

    DELETE FROM emp e

    WHERE EXISTS

    (SELECT 'empno'

    FROM emp2

    WHERE empno = e.empno);

    UPDATE emp e

    SET ename =

    (SELECT ename

    FROM emp2

    WHERE empno = e.empno);

  • 相关阅读:
    Git本地库在哪
    Git&GitHub-添加提交以及查看状态
    linux命令——find
    正则表达式
    再访JavaScript对象(原型链和闭包)
    RabbitQM(消息队列)
    Java泛型(T)与通配符?
    Linux设置文件权限和归属
    英语单词
    RabbitQM使用笔记
  • 原文地址:https://www.cnblogs.com/cnblogs5359/p/12053127.html
Copyright © 2011-2022 走看看