sql - 在 Mariadb 中，我对多个表的选择查询返回过多的行

Question

我对 Maria db 很陌生，但我还没有找到解决这个问题的方法。我有一个包含几个表的数据库。其中两张表是员工和客户。他们彼此没有任何关系。

create table clients (id int, name varchar(50), city varchar (20))
create table workers (id int, name varchar (50), city varchar(20))

我正在尝试编写一个选择查询，它将返回一个城市的名称、工人总数以及该城市的客户总数。clients 表有超过 2000 个条目，workers 表有 60 个。

问题是，如果我这样做：

Select workers.id, clients.id from workers, clients; 

我得到 120,000 行（60 * 2000）。我如何实现我的目标？

很抱歉，如果格式很奇怪。

Answer 1

您可以使用union all和group by：

select city, sum(num_clients) as num_clients,
       sum(num_workers) as num_workers
from ((select city, count(*) as num_clients, 0 as num_workers
       from clients 
       group by city
      ) union all
      (select city, 0 as num_clients, count(*) as num_workers
       from workers 
       group by city
      ) 
     ) c
group by city;

Answer 2

这就是你需要的。

select
  t1.city,
  sum(t1.workers),
  sum(t1.clients)
from
  (
    select
      city,
      1 as 'workers',
      0 as 'clients'
    from
      workers
    union all
    select
      city,
      0 as 'workers',
      1 as 'clients'
    from
      clients
  ) t1
group by
  t1.city;