前端初学 SQL，求帮忙修改一个简单 MySQL 联查语句

V2EX = way to explore

V2EX 是一个关于分享和探索的地方

For Existing Member Sign In

This topic created in 1281 days ago, the information mentioned may be changed or developed.

```
SELECT A.ItemId, Description, CreatedAt, B.GroupTitle FROM items A, groups B WHERE A.GroupId = B.GroupId AND UserId = 1 ORDER BY ItemId
```

这个查询会导致如果 A 的 GroupId 为 NULL ，A 这条记录就会被忽略掉。我想要的效果是该记录 GroupTitle 变成 NULL 或者空字符串，剩下的字段都在，研究了很久都没成功。

5 replies • 2022-11-04 10:59:33 +08:00

PendingOni

Nov 4, 2022

A 去 left join B?
```SQL
SELECT A.ItemId, Description, CreatedAt, B.GroupTitle
FROM items A
left join
groups B on A.GroupId = B.GroupId
WHERE UserId = 1
ORDER BY ItemId
```

kkwa56188

Nov 4, 2022

-- 也可以用子查询. 这里用了个 max()是怕 B.GroupId 不唯一
SELECT A.ItemId, A.Description, A.CreatedAt
, ( select max(B.GroupTitle) from groups B WHERE B.GroupId = A.GroupId )
FROM items A
AND A.UserId = 1
ORDER BY A.ItemId;

akira

Nov 4, 2022

`
SELECT
A.ItemId,
A.Description,
A.CreatedAt,
B.GroupTitle
FROM items A
left join groups B on A.GroupId = B.GroupId
WHERE 1=1
AND UserId = 1
ORDER BY ItemId
`

xuelu520

Nov 4, 2022

left join 就行。
楼主可以去看看 left/right/inner 几种的区别，几种 join 用的还是很多的。

edis0n0

Nov 4, 2022

成功了，谢谢大家