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  • SQL count命令,avg命令

    with student as(
    select 1 as id from dual union all
    select null as id from dual union all
    select 3 as id from dual 
    )
    
    --如果使用count(1)则计算所有,如果列出的使用count(列名)则会滤掉null值
    select count(id), count(1),count(*) from student

    with student as(
    select 1 as id from dual union all
    select null as id from dual union all
    select 3 as id from dual 
    )
    select min(id), max(id) from student;

    with student as(
    select 1 as id from dual union all
    select null as id from dual union all
    select 3 as id from dual union all
    select 3 as id from dual 
    )
    select count(distinct id) from student;

    这里在count中看到的是【去掉null 以后,非重复的内容】

    -------------------------------------------------------------------------------------------------

    with student as(
    select 1 as id from dual union all
    select null as id from dual union all
    select 3 as id from dual union all
    select 3 as id from dual 
    )
    select avg(id) from student;

    结果是7/3的结果,即滤掉NULL以后,使用(1+3+3)/3作为AVG的运算方式

    但换个方法

    with student as(
    select 1 as id from dual union all
    select null as id from dual union all
    select 3 as id from dual union all
    select 3 as id from dual 
    )
    select avg(nvl(id,0)) from student;

    就是以4为除数了。

     那么,对于【组函数】我们可以使用下面这一个语句

    with student as(
    select 1 as id from dual union all
    select null as id from dual union all
    select 3 as id from dual union all
    select 3 as id from dual 
    )
    select min(id),max(id),avg(nvl(id,0)),count(1) from student;
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  • 原文地址:https://www.cnblogs.com/adamgq/p/14319665.html
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