mysql – 了解自我加入
作者：互联网
我正在练习自我加入,这是我在编写查询时不理解的事情.
我有一张桌子’员工’
employee表包含三条记录.
+-----+---------------+------------+
| id | employee | manager_id |
+-----+---------------+------------+
| 1 | Ola | NULL |
| 2 | Ahmed | 1 |
| 3 | Tove | 1 |
+----------+----------+------------+
最后一列manager_id指的是第一列id,他是Ahmed和Tove的Ola经理.
如果我写的查询像
SELECT emp.employee as NAME, manager.employee as MANAGER
FROM employee as emp, employee as manager
WHERE emp.id = manager.manager_id
结果使艾哈迈德和托夫经理.
而
SELECT emp.employee as NAME, manager.employee as MANAGER
FROM employee as emp, employee as manager
WHERE manager.id = emp.manager_id
使它正确,有人可以解释一下吗？
解决方法:
自联接就像一个内部联接,其中同一个表的两个或多个实例通过公共数据类型列/字段连接在一起.这种连接(内连接)根据连接条件给出公共行.
employee表包含三条记录.在这种情况下,
员工为emp：
+-----+---------------+------------+
| id | employee | manager_id |
+-----+---------------+------------+
| 1 | Ola | NULL |
| 2 | Ahmed | 1 |
| 3 | Tove | 1 |
+----------+----------+------------+
员工担任经理：
+-----+---------------+------------+
| id | employee | manager_id |
+-----+---------------+------------+
| 1 | Ola | NULL |
| 2 | Ahmed | 1 |
| 3 | Tove | 1 |
+----------+----------+------------+
现在第一个案例：让我们尝试这个来理解差异：
SELECT emp.*,经理.*
员工为emp,员工为经理
WHERE emp.id = manager.manager_id
+-----+---------------+------------+-----+---------------+------------+
| id | employee | manager_id | id | employee | manager_id |
+-----+---------------+------------+-----+---------------+------------+
| 1 | Ola | NULL | 2 | Ahmed | 1 |
| 1 | Ola | NULL | 3 | Tove | 1 |
+----------+----------+------------+----------+----------+------------+
请参阅emp.id = manager.manager_id.因此,作为NAME的emp.employee从第一个表&给出了Ola行.作为经理的manager.employee给了Ahmed&从第二张桌子的托夫.
现在第二个案例：让我们试试这个来理解它们的区别：
SELECT emp.*,经理.*
员工为emp,员工为经理
WHERE manager.id = emp.manager_id
+-----+---------------+------------+-----+---------------+------------+
| id | employee | manager_id | id | employee | manager_id |
+-----+---------------+------------+-----+---------------+------------+
| 2 | Ahmed | 1 | 1 | Ola | NULL |
| 3 | Tove | 1 | 1 | Ola | NULL |
+----------+----------+------------+----------+----------+------------+
请参阅manager.id = emp.manager_id.因此,作为NAME的emp.employee给出了Ahmed&的行.第一桌和第二桌manager.employee作为MANAGER从第二个表中给出Ola行.
标签：self-join,sql,mysql 来源： https://codeday.me/bug/20190831/1772201.html