Question

我有成员表

table members :
pid|  id    | name
1  |  id01  | jenny
2  |  id02  | kain
3  |  id03  | alex

并有另一个表members_opt

table members_opt
pid | members_id | category
1   | id01       | cat
2   | id01       | dog
3   | id02       | dog
4   | id03       | NULL

现在我使用下面的SQL查询

SELECT * FROM members a JOIN
 (SELECT members_id, max(category) as category FROM members_opt GROUP BY members_id) b
ON a.id = b.members_id

但是这个SQL Query没有捕获“id03”的数据，因为“id03”的members_opt.category是NULL

我想要这个结果

result : 
id   | name  | category
id01 | jenny | cat
id02 | kain  | dog
id03 | alex  | NULL

（结果现在显示双重名称，双重id值。）

我如何使用SQL查询？

Answer 1

你可以试试这个 -

SELECT * FROM成员JOIN （SELECT members_id，max（类别为IS NULL，然后为0）作为类别FROM members_opt GROUP BY members_id）b ON a.id = b.members_id

Answer 2

使用以下查询获得所需的输出 -

SELECT a.id, a.name, b.category FROM members a INNER JOIN
 (SELECT members_id, category FROM members_opt GROUP BY members_id) b
ON a.id = b.members_id

Answer 3

只需将JOIN更改为LEFT JOIN：

SELECT a.id, a.name, b.category
FROM members a
LEFT JOIN (
    SELECT members_id, max(category) as category
    FROM members_opt GROUP BY members_id) b
ON a.id = b.members_id

如果你想'cat'代表'jenny'，你应该使用聚合函数min。

MySQL JOIN，WHERE和IF问题

3 个答案: