PostgreSQL：当x = a和x = b时,如何从表中选择2个不同的计数

亲爱的朋友Stackoverflowers,

如何从场景中的同一个表中选择2个不同的计数：

x=a and x=b

具体来说,(WHEN type = subdomain)AND(WHEN subtype = subdomain)？

将它们添加到一起以创建’totalcount’？

我的尝试(供你参考)：

SELECT description,type,count(1),subtype,count(2) 
FROM mapping,(dotcom WHERE type = subdomain) AS typecount,(dotcom WHERE subtype = subdomain) AS subtypecount 
GROUP BY description,subtype 
HAVING count(1)>1 
AND count(2)>1 
ORDER BY count(*) 
DESC LIMIT 10

第二次尝试：

SELECT description
FROM mapping,SUM(WHEN subdomain = type OR subdomain = subtype) AS count(1)
GROUP BY description,subtype 
HAVING count(1)>1 
ORDER BY count(*) 
DESC LIMIT 10

解决方法

如果你有Postgres 9.4或更新版本,你可以使用FILTER,它比CASE WHEN … END语法更容易阅读.

SELECT
  description,COUNT(*) FILTER (WHERE subdomain = type) AS typecount,COUNT(*) FILTER (WHERE subtype = subdomain) AS subtypecount
FROM mapping
GROUP BY description,subtype;

应该注意的是,count(1)和count(2)并不像你想象的那样; count是一个聚合函数,它计算每一行中给定值是否为空,并且看到你给它一个整数字面值,它实际上只计算返回的行数.