Question

我必须从三个表avg_salary，人员和电子邮件执行查询。这个简单的SQL查询工作正常。

SELECT avg_salary.id, COUNT(emails.message_from) AS email_PGA
FROM avg_salary, person, emails
WHERE person.works_in = avg_salary.id    
AND person.email_address = emails.message_from
AND person.salary::numeric > avg_salary.avg
GROUP BY avg_salary.id

但是我希望在条件时添加另一列email_PLA person.salary::numeric < avg_salary.avg。我可以通过再次加入整个查询来做到这一点。但我希望在这种情况下使用CASE。即使经过这么多次尝试，我也无法正确理解语法。

Answer 1

我认为你需要另一个计数？你需要这样的东西：

SUM(CASE WHEN (person.salary::numeric < avg_salary.avg) THEN 1 ELSE 0 END) AS email_PGA

Answer 2

您可以使用案例expression执行条件聚合，并始终使用显式JOIN语法

SELECT avg_salary.id, 
       SUM(CASE WHEN p.salary::numeric > asal.avg THEN 1 ELSE 0 END) AS email_PGA,
       SUM(CASE WHEN p.salary::numeric < asal.avg THEN 1 ELSE 0 END) AS email_PLA
FROM avg_salary asal, 
INNER JOIN person  p on p.works_in = asal.id
INNER JOIN emails e on e.message_from = p.email_address  
--WHERE p.salary::numeric > asal.avg
GROUP BY avg_salary.id;

Answer 3

如果在特定条件下需要不同的列，则必须执行不同的SQL查询。

带有case语句的sql count查询

3 个答案: