我有三个表:users
,items
和boards
。
create table users (
id serial primary key,
name text
);
create table items (
id serial primary key,
user_id int,
title text
);
create table boards (
id serial primary key,
user_id int,
title text
);n
我想选择具有每个用户拥有的项目和板数的所有用户:
select
users.*,
count(items) as item_count,
count(boards) as board_count
from users
left join items on items.user_id = users.id
left join boards on boards.user_id = users.id
group by users.id;
不幸的是,这并没有给我正确的数量。计数高于应有的数量。发生了什么,为什么,以及如何解决这个问题?
答案 0 :(得分:7)
问题是由两个左连接产生重复值的事实引起的。
要解决此问题,您可以只计算不同的值
SELECT u.id, u.name,
COUNT(DISTINCT i.id) item_count,
COUNT(DISTINCT b.id) board_count
FROM users u LEFT JOIN items i
ON u.id = i.user_id LEFT JOIN boards b
ON u.id = b.user_id
GROUP BY u.id, u.name
或单独计算每个用户的项目和板子,然后将其与用户联系
SELECT u.*,
COALESCE(i.item_count, 0) item_count,
COALESCE(b.board_count, 0) board_count
FROM users u LEFT JOIN
(
SELECT user_id, COUNT(*) item_count
FROM items
GROUP BY user_id
) i ON u.id = i.user_id LEFT JOIN
(
SELECT user_id, COUNT(*) board_count
FROM boards
GROUP BY user_id
) b ON u.id = b.user_id
这是 SQLFiddle 演示
答案 1 :(得分:3)
我认为这应该可以解决问题
select
u.*,
(select count(*) from items where user_id = u.id) as item_count,
(select count(*) from boards where user_id = u.id) as board_count
from users as u
group by u.id;