大家好，我在MySQL中有4个表作为键值存储
t1 (article): t2:
| id | date | | id | key | value |
------------- ---------------------------
| 1 | 2016 | | 1 | title | title1 |
| 2 | 2017 | | 1 | user_id | 1 |
| 3 | 2018 | | 2 | title | title2 |
------------- | 2 | user_id | 2 |
| 3 | title | title3 |
| 3 | user_id | 1 |
---------------------------
t1 (user): t2:
| id | date | | id | key | value |
------------- -------------------------
| 1 | NULL | | 1 | name | user1 |
| 2 | NULL | | 2 | name | user2 |
------------- -------------------------
SELECT t1.id,
GROUP_CONCAT(IF(t2.key='title',t2.value,NULL)) AS title,
t1.date,
GROUP_CONCAT(IF(t2.key='user_id',t2.value,NULL)) AS user_id,
(
SELECT GROUP_CONCAT(IF(t4.key='user_name',t4.value,NULL))
FROM t4
GROUP BY t4.id
HAVING t4.id = user_id
) AS user_name
FROM t1
INNER JOIN t2
ON t1.id = t2.id
GROUP BY t1.id
我想打印出存储在t2中作为id的用户名，例如：
| id | title | date | user_id | user_name |
------------------------------------------------
| 1 | title1 | 2016 | 1 | user1 |
| 2 | title2 | 2017 | 2 | user2 |
| 3 | title3 | 2018 | 1 | user1 |
------------------------------------------------
我已经测试了WHERE子句和HAVING子句，但是对我没有任何作用。
最佳答案
我发现您的表引用太混乱了，因此我使用了示例数据的解释。顺便说一下，我只需要4张桌子中的3张。 Demo
MySQL 5.6模式设置：
CREATE TABLE articles
(`id` int, `date` date)
;
INSERT INTO articles
(`id`, `date`)
VALUES
(1, '2016-01-01'),
(2, '2017-01-01'),
(3, '2018-01-01')
;
CREATE TABLE users
(`id` int, `date` date)
;
INSERT INTO users
(`id`, `date`)
VALUES
(1, NULL),
(2, NULL)
;
CREATE TABLE t2_upper
(`id` int, `key` varchar(7), `value` varchar(6))
;
INSERT INTO t2_upper
(`id`, `key`, `value`)
VALUES
(1, 'title', 'title1'),
(1, 'user_id', '1'),
(2, 'title', 'title2'),
(2, 'user_id', '2'),
(3, 'title', 'title3'),
(3, 'user_id', '1')
;
CREATE TABLE t2_lower
(`id` int, `key` varchar(4), `value` varchar(5))
;
INSERT INTO t2_lower
(`id`, `key`, `value`)
VALUES
(1, 'name', 'user1'),
(2, 'name', 'user2')
;
查询1：
select a.id, tn.value article_title, a.date, tu.id user_id, u.value user_name
from articles a
inner join (
select
*
from t2_upper
where `key` = 'title'
) tn on a.id = tn.id
inner join (
select
*
from t2_upper
where `key` = 'user_id'
) tu on a.id = tu.id
inner join (
select
*
from t2_lower
where `key` = 'name'
) u on tu.value = u.id
Results：
| id | article_title | date | user_id | user_name |
|----|---------------|------------|---------|-----------|
| 1 | title1 | 2016-01-01 | 1 | user1 |
| 2 | title2 | 2017-01-01 | 2 | user2 |
| 3 | title3 | 2018-01-01 | 3 | user1 |
关于php - 在子查询中使用外部别名与mysql中的4个表，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/48334084/