请看下面的查询 - 我得到Unknown column 'u.id' in 'on clause'
SELECT id, username,
coalesce(
(SELECT name from company c
INNER JOIN user_company uc
ON uc.user_id = u.id
AND c.id = uc.company_id), 'NOT-AVAILABLE'
) companyname
FROM `user` u
请看下面的查询 - 我得到Unknown column 'u.id' in 'on clause'
SELECT id, username,
coalesce(
(SELECT name from company c
INNER JOIN user_company uc
ON uc.user_id = u.id
AND c.id = uc.company_id), 'NOT-AVAILABLE'
) companyname
FROM `user` u
根据您的评论,相关性不能放在JOIN
相关子查询的范围内。
但是,它可以放在WHERE
相关子查询的子句中。
SELECT
id,
username,
coalesce(
(SELECT name
FROM company c
INNER JOIN user_company uc
ON c.id = uc.company_id
WHERE uc.user_id = u.id
),
'NOT-AVAILABLE'
) companyname
FROM
`user` u
这回答了你的明确问题;为什么您的查询在语法上失败。
但是,我会用一个简单的LEFT JOIN
.
SELECT
u.id,
u.username,
COALESCE(c.name, 'NOT-AVAILABLE') companyname
FROM
`user` u
LEFT JOIN
`user_company` uc
ON uc.user_id = u.id
LEFT JOIN
`company` c
ON c.id = uc.company_id
一种解决方法:
SELECT u.id,
u.username,
COALESCE(aux.name, 'NOT-AVAILABLE') as 'companyname'
FROM `user` u
LEFT JOIN
(SELECT user_id, name from company c
INNER JOIN user_company uc
ON c.id = uc.company_id) aux ON aux.user_id = u.id
另一种解决方法:
SELECT u.id,
u.username,
COALESCE(c.name, 'NOT-AVAILABLE') AS 'companyname'
FROM `user` u
LEFT JOIN user_company uc ON uc.user_id = u.id
LEFT JOIN company c ON c.id = uc.company_id