Question

我有三张桌子，公司，用户和分享。我想算一个公司的用户和分享，他们不相关。

可能有一行具有共享值但不具有用户值。所以我使用左连接，我可以单独得到结果，但它不能一起工作。

这是我的问题：

SELECT c.name, count(u.company_id), count(s.company_id)
FROM company c
LEFT JOIN user u
ON c.id=u.company_id and u.company_id=337
WHERE u.company_id is NOT NULL 
LEFT JOIN share s
ON c.id=s.id AND s.company_id=337 
WHERE s.company_id is NOT NULL

Answer 1

您需要在子查询中至少执行一个计数。否则，两个计数都是相同的，因为您只是计算得到的叉积中的行。

SELECT c.name, user_count, share_count
FROM company AS c
JOIN (SELECT company_id, COUNT(*) AS user_count
      FROM users
      GROUP BY company_id) AS u
ON u.company_id = c.id
JOIN (SELECT company_id, COUNT(*) AS share_count
      FROM share 
      GROUP BY company_id) AS s
ON s.company_id = c.id
WHERE c.company_id = 337

另一种选择是计算您加入的表的不同主键：

SELECT c.name, COUNT(DISTINCT u.id) AS user_count, COUNT(DISTINCT s.id) AS share_count
FROM company AS c
JOIN users AS u on u.company_id = c.id
JOIN share AS s ON s.company_id = c.id
WHERE c.company_id = 337

Answer 2

除了额外的WHERE子句外，您的代码看起来还可以。但是，您可能需要COUNT(DISTINCT)，因为这两个计数将返回相同的值：

SELECT c.name, count(distinct u.company_id), count(distinct s.company_id)
FROM company c LEFT JOIN
     user u
     ON c.id = u.company_id and u.company_id=337 LEFT JOIN
     share s
     ON c.id = s.id AND s.company_id=337 
WHERE s.company_id is NOT NULL AND u.company_id IS NOT NULL;

使用2左连接无法工作，但可以单独获得结果

2 个答案: