我有一堆应聘者,他们使用某种技能,都有一份或多份工作,每个人在一家公司工作。
不良的ascii艺术如下:
--------------- ---------------
| candidate 1 | | candidate 2 |
--------------- \ --------------
/ \ |
------- -------- etc
|job 1| | job 2 |
------- ---------
/ \ / \
--------- --------- --------- --------
|company | | skills | |company | | skills |
--------- --------- ---------- ----------
这是我的数据库:
mysql> describe jobs;
+--------------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+---------+------+-----+---------+----------------+
| job_id | int(11) | NO | PRI | NULL | auto_increment |
| candidate_id | int(11) | NO | MUL | NULL | |
| company_id | int(11) | NO | MUL | NULL | |
| start_date | date | NO | MUL | NULL | |
| end_date | date | NO | MUL | NULL | |
+--------------+---------+------+-----+---------+----------------+
。
mysql> describe candidates;
+----------------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+----------+------+-----+---------+----------------+
| candidate_id | int(11) | NO | PRI | NULL | auto_increment |
| candidate_name | char(50) | NO | MUL | NULL | |
| home_city | char(50) | NO | MUL | NULL | |
+----------------+----------+------+-----+---------+----------------+
。
mysql> describe companies;
+-------------------+---------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------------+---------------+------+-----+---------+----------------+
| company_id | int(11) | NO | PRI | NULL | auto_increment |
| company_name | char(50) | NO | MUL | NULL | |
| company_city | char(50) | NO | MUL | NULL | |
| company_post_code | char(50) | NO | | NULL | |
| latitude | decimal(11,8) | NO | | NULL | |
| longitude | decimal(11,8) | NO | | NULL | |
+-------------------+---------------+------+-----+---------+----------------+
。
mysql> describe skills;
+----------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------+------+-----+---------+-------+
| skill_id | int(11) | NO | MUL | NULL | |
| job_id | int(11) | NO | MUL | NULL | |
+----------+---------+------+-----+---------+-------+
。
mysql> describe skill_names;
+------------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+----------+------+-----+---------+----------------+
| skill_id | int(11) | NO | PRI | NULL | auto_increment |
| skill_name | char(32) | NO | MUL | NULL | |
+------------+----------+------+-----+---------+----------------+
这是我的查询尝试(请注意,我打算将通配符更改为字段名称;我只是想使某些功能生效):
SELECT can.* , co.*, j.*, sn.*
FROM candidates AS can
JOIN jobs AS j
JOIN companies AS co ON j.company_id = co.company_id
JOIN skills AS s ON s.job_id = j.job_id
JOIN skill_names AS sn ON s.skill_id = s.skill_id
HediSql说/* Affected rows: 0 Found rows: 34,461,651 Warnings: 0 Duration for 1 query: 0.000 sec. (+ 105.078 sec. network) */
查询出了什么问题?我希望从不良的ascii艺术中可以清楚地知道我要达到的目标。
(此外,加入表的顺序是否会造成速度差异?我会担心新的MySql v8函数会在以后将其作为嵌套JSON检索)
答案 0 :(得分:5)
您缺少candidates
和jobs
之间的联接条件,因此在两个表之间都得到了笛卡尔积。另外,skill_names
上的连接条件存在问题,其中两列都相同(这再次生成笛卡尔积)。
SELECT can.* , co.*, j.*, sn.*
FROM candidates AS can
JOIN jobs AS j ON j.candidate_id = can.candidate_id --> here: missing join condition
JOIN companies AS co ON j.company_id = co.company_id
JOIN skills AS s ON s.job_id = j.job_id
JOIN skill_names AS sn ON sn.skill_id = s.skill_id --> and here: wrong join condition
许多RDBMS会在没有JOIN
子句的ON
上引发语法错误(如果您确实想要笛卡尔积,则需要使用CROSS JOIN
对其进行明确说明),但是,a,不是MySQL。
关于这个问题:
我加入桌子的顺序对速度有什么影响?
不。只要您使用的是inner join
(而不是left join
),查询计划者的连接顺序就无关紧要,它将以它认为更有效的顺序重新排列它们。>