Question

有人请帮帮我。

我有2个表，我的数据库是MySQL

表1

user_id | username
------- | --------
1       | joni
2       | adam
3       | jerry
4       | Dino

表2

dokumen_id | create_by | update_by | last_access_by |
doc_001    | 2         | 1         | 2              |
doc_002    | 3         | 2         | 1              |
doc_003    | 1         | 1         | 4              |

我想在一个查询中加入2个表，结果如下

dokumen_id | create_by | update_by | last_access_by |
doc_001    | adam      | joni      | adam           |
doc_002    | jerry     | adam      | joni           |
doc_003    | joni      | joni      | dino           |

我该如何编写查询？

非常感谢你的帮助。

Answer 1

您需要加入table1三次

SELECT t2.dokumen_id,
       c.username AS create_by,
       u.username AS update_by,
       l.username AS last_access_by
FROM   table2 t2
       LEFT JOIN table1 c
              ON t2.create_by = c.user_id
       LEFT JOIN table1 u
              ON t2.update_by = u.user_id
       LEFT JOIN table1 l
              ON t2.last_access_by = l.user_id

左外部联接用于返回table2中的所有记录，用户可能在create_by中但不在last_access_by中，在这种情况下，内部联接将过滤记录不在create_by

中

Answer 2

使用table1

加入table2三次

select t1.dokumen_id,
  tc.username create_by,
  tu.username update_by,
  ta.username last_access_by
from table2 t1
left outer join table1 tc
  on tc.user_id = t1.create_by
left outer join table1 tu
  on tu.user_id = t1.update_by
left outer join table1 ta
  on ta.user_id = t1.last_access_by;

Answer 3

SELECT * FROM table1
LEFT JOIN table2
ON table1.user_id=table2.last_access_by;

Answer 4

你需要加入table1三次

SELECT t2.dokumen_id,
       c.username AS create_by,
       u.username AS update_by,
       l.username AS last_access_by
FROM   table2 t2
       LEFT JOIN table1 c
              ON t2.create_by = c.user_id
       LEFT JOIN table1 u
              ON t2.update_by = u.user_id
       LEFT JOIN table1 l
              ON t2.last_access_by = l.user_id

JOIN 2 SQL表（表1中的1个字段替换另一个表中的1个以上字段）

4 个答案: