MySql 自 JOIN 查询 [英] MySql self JOIN query

问题描述

我有这张桌子

mysql> describe skill_usage;
+----------+---------+------+-----+---------+-------+
| Field    | Type    | Null | Key | Default | Extra |
+----------+---------+------+-----+---------+-------+
| skill_id | int(11) | NO   | MUL | NULL    |       |
| job_id   | int(11) | NO   | MUL | NULL    |       |
+----------+---------+------+-----+---------+-------+

并且知道在我的数据中，有一个 job_id (6) 用于 skill_id 3 和 4:

and know that in my data, there is a single job_id (6) which was used for both skill_id 3 and 4:

mysql>  select * from skill_usage;
+----------+--------+
| skill_id | job_id |
+----------+--------+
|        1 |      1 |
|        2 |      2 |
|        3 |      3 |     <----  matches only one part of the AND clause
|        3 |      4 |     <----  matches only one part of the AND clause
|        2 |      5 |
|        3 |      6 |     <==== matches both parts of the AND clause
|        4 |      6 |     <====
|        2 |      7 |
+----------+--------+
8 rows in set (0.00 sec)

这是我尝试过的:

SELECT DISTINCT s1.job_id FROM skill_usage AS s1 
  INNER JOIN skill_usage AS s2 ON s1.job_id = s2.job_id
    WHERE s1.skill_id IN (3,4)
    AND   s2.skill_id IN (3,4)

我认为这意味着找到所有与 skill_id 3 和 skill_id 4 匹配的job_id".

which I thought meant "find all job_id which matches both skill_id 3 and skill_id 4".

显然不是:

mysql> SELECT DISTINCT s1.job_id FROM skill_usage AS s1
    ->   INNER JOIN skill_usage AS s2 ON s1.job_id = s2.job_id
    ->     WHERE s1.skill_id IN (3,4)
    ->     AND   s2.skill_id IN (3,4);
+--------+
| job_id |
+--------+
|      3 |
|      4 |
|      6 |
+--------+
3 rows in set (0.00 sec)

我做错了什么?我的查询应该如何阅读?我认为是时候写一本好书或 Udemy 课程了，但我没有自己的封面自我加入.

What am I doing wrongly? How should my query read? I think that it's time for a good book or Udemy course, but none that I own cover self join.

我的查询正确地找到了 job_id = 6，但错误地(IMO)找到了 job_id 3 和 4.我希望它们失败 AND 子句.

My query is correctly finding job_id = 6, but wrongly (IMO), finding job_id 3 and 4. I would expect them to fail the AND clause.

推荐答案

替代解决方案.
(参考 db fiddle)

select s1.job_id
  from skill_usage s1
  where s1.skill_id = 3
    and s1.job_id in (
                       select s2.job_id
                         from skill_usage s2
                        where s2.skill_id = 4
                     )

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