Question

我正在尝试找出一个SQL查询以一次从多个表中提取数据。香港专业教育学院有一张桌子，上面有公司清单。我还有另外两个表，其中包含每个公司的通讯。目前，我正在遍历每个公司行，然后对每个通讯表执行查询。我试过像这样的查询：

SELECT c.*, 
       count(e.id) AS ecount, 
       count(p.id) AS pcount 
FROM companies c 
LEFT JOIN emails e 
  ON c.id = e.company 
LEFT JOIN push p 
  ON c.id = p.company

但是此查询的结果仅包含一个公司，没有计数。如何完成此查询？

Answer 1

您可能打算：

SELECT c.*, 
       count(distinct e.id) AS ecount, 
       count(distinct p.id) AS pcount 
FROM companies c LEFT JOIN
     emails e 
     ON c.id = e.company LEFT JOIN
     push p
     ON c.id = p.company
GROUP BY c.id;

假设companies.id是唯一键或主键，那么这是表达逻辑的有效方法。

Answer 2

SELECT c.*, 
       (SELECT COUNT(e.id) FROM emails e WHERE e.company = c.id) as ecount,
       (SELECT COUNT(p.id) FROM push p   WHERE p.company = c.id) as pcount 
FROM companies c

Answer 3

SELECT c.*, 
       ( SELECT count(e.id) FROM emails WHERE e.company = c.id ) AS ecount, 
       ...
    FROM companies c

多表左联接

3 个答案: