使用计数子查询、内部联接和组进行查询 [英] query with count subquery, inner join and group
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问题描述
我绝对是 SQL 的菜鸟,我一直在拼命在 Postgresql 中编写一个具有以下表结构的复杂查询:
I'm definitely a noob with SQL, I've been busting my head to write a complex query with the following table structure in Postgresql:
CREATE TABLE reports
(
reportid character varying(20) NOT NULL,
userid integer NOT NULL,
reporttype character varying(40) NOT NULL,
)
CREATE TABLE users
(
userid serial NOT NULL,
username character varying(20) NOT NULL,
)
查询的目的是获取每个用户的报告类型数量并将其显示在一列中.共有三种不同类型的报告.
The objective of the query is to fetch the amount of report types per user and display it in one column. There are three different types of reports.
使用 group-by 的简单查询将解决问题,但将其显示在不同的行中:
A simple query with group-by will solve the problem but display it in different rows:
select count(*) as Amount,
u.username,
r.reporttype
from reports r,
users u
where r.userid=u.userid
group by u.username,r.reporttype
order by u.username
推荐答案
SELECT
username,
(
SELECT
COUNT(*)
FROM reports
WHERE users.userid = reports.userid && reports.reporttype = 'Type1'
) As Type1,
(
SELECT
COUNT(*)
FROM reports
WHERE users.userid = reports.userid && reports.reporttype = 'Type2'
) As Type2,
(
SELECT
COUNT(*)
FROM reports
WHERE users.userid = reports.userid && reports.reporttype = 'Type3'
) As Type3
FROM
users
WHERE
EXISTS(
SELECT
NULL
FROM
reports
WHERE
users.userid = reports.userid
)
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