sql - 如何连接多个表进行计数？

Question

我有 3 个表，分别名为 houses、trees、rivers。所有这些表都有 city_id 列。我想按城市对总数进行分组。城市在另一个表中。 我的数据库是postgresql。

city_name    trees    houses      rivers
City-1       1000     200         1
City-2       300      100         2
City-3       4000     210         4

我可以去找树

SELECT 
    city.name as city_name, 
    count(*) as trees
FROM trees as t, cities as city
WHERE t.city_id = city.city_id
GROUP BY city.name

但是我无法在 sama 查询中连接三个表。

Answer 1

为了避免 JOIN 中的行重复问题，最简单的方法可能是在子查询中进行聚合，然后 JOIN 它们:

SELECT c.name, 
       COALESCE(t.cnt, 0) AS trees,
       COALESCE(h.cnt, 0) AS houses,
       COALESCE(r.cnt, 0) AS rivers
FROM cities c
LEFT JOIN (SELECT city_id, COUNT(*) AS cnt
           FROM trees
           GROUP BY city_id) t ON t.city_id = c.city_id
LEFT JOIN (SELECT city_id, COUNT(*) AS cnt
           FROM houses
           GROUP BY city_id) h ON h.city_id = c.city_id
LEFT JOIN (SELECT city_id, COUNT(*) AS cnt
           FROM rivers
           GROUP BY city_id) r ON r.city_id = c.city_id

如果给定城市没有树木、房屋或河流，我们使用LEFT JOIN。

Demo on dbfiddle