带有条件条件语句的总和在SQL中如何工作 [英] How sum with case conditional statement works in sql

问题描述

前几天，我回答了这个问题，但是其他用户使用sum + case条件语句解决了该问题，并在结果中添加了一个边缘条件.因此，我想到了一个问题，以下查询中的sum(case when jobname = 'Analyst' then 1 else 0 end)语句是如何工作的

The other day, I gave an answer to this question but then other user solved that problem with sum + case conditional statement to add one edge condition in result. So, the question came to my mind, how statement sum(case when jobname = 'Analyst' then 1 else 0 end) in the below query works

select d.*
from (select deptno,
         sum(case when jobname = 'Analyst' then 1 else 0 end) as numAnalysts
      from employees
      group by deptno
      order by numAnalysts asc
     ) d
where rownum = 1;`

，并返回部门中的员工人数.另外，我想了解此查询的性能.

and return the number of employees over a department. Also, I would like to understand the performance of this query.

在发布此问题之前，我阅读了 this ， this 和此，但仍然没有无法了解其工作原理.

Before posting this question, I read this, this and this but still didn't get how this works.

推荐答案

大概这是您在努力理解的部分:

Presumably, this is the part that you are struggling to understand:

  select deptno,
         sum(case when jobname = 'Analyst' then 1 else 0 end) as numAnalysts
  from employees
  group by deptno

这确实是一个简单的聚合查询.该查询正在执行的操作是:

This is a simple aggregation query, really. What the query is doing is:

• 查看employees
中的每一行
• 如果jobname是'Analyst'，则分配1的值(这是case语句. Otherwise, assign a value of 0`.
• 按部门汇总，将刚刚计算出的值相加.这具有计算分析师人数的作用.

• Look at each row in employees
• If jobname is 'Analyst' then assign the value of 1 (this is the case statement. Otherwise, assign a value of0`.
• Aggregate by department, summing the value just calculated. This has the effect of counting the number of analysts.

case是一个返回值的表达式. sum()只是将每个组的值相加.

case is an expression that returns a value. The sum() is simply adding up that value for each group.

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