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  • Oracle中查询关键字select--from--where--group by--having--order by执行顺序

         select--from--where--group by--having--order by 这6个查询关键字的执行顺序:

    1.from组装来自不同数据源的数据;
    2.where基于指定的条件对记录行进行筛选;
    3.group by子句将数据划分为多个分组;
    4.使用聚集函数进行计算;
    5.使用having子句筛选分组;
    6.计算所有的表达式;
    7.select 集合输出;
    8.使用order by对结果集进行排序。

     

    以下例子基于Oracle自带表空间hr

    例子1:having和select的执行说明

    情景:关于哪个部门员工最多?

    A

    1 SELECT D1.DEPARTMENT_ID,D2.DEPARTMENT_NAME
    2 FROM EMPLOYEES D1 JOIN DEPARTMENTS D2 ON D1.DEPARTMENT_ID=D2.DEPARTMENT_ID
    3 GROUP BY D1.DEPARTMENT_ID ,D2.DEPARTMENT_NAME
    4 HAVING count(*)>= all(SELECT count(*) FROM EMPLOYEES GROUP BY DEPARTMENT_ID)

    B

    1 SELECT D1.DEPARTMENT_ID,D2.DEPARTMENT_NAME,count(*)
    2 FROM EMPLOYEES D1 JOIN DEPARTMENTS D2 ON D1.DEPARTMENT_ID=D2.DEPARTMENT_ID
    3 GROUP BY D1.DEPARTMENT_ID ,D2.DEPARTMENT_NAME
    4 HAVING count(*)>= all(SELECT count(*) FROM EMPLOYEES GROUP BY DEPARTMENT_ID)

    A、B结果相同,说明having先于select执行

     

     

     

     

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  • 原文地址:https://www.cnblogs.com/Deribs4/p/5095424.html
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