如何在mysql中不使用完全外部连接就实现这一点？

我有以下表格：

1）

customfields

+------+---------+-------+
|userid| fieldid | value |
+------+---------+-------+
|  1   |   1     |  5c54 |
|  2   |   1     |  cerc |
|  2   |   3     |  3545 |

+--------+---------+-------+
|name    | piva    | code  |
+--------+---------+-------+
|  ale   |   5c54  |       |
|  ben   |   cerc  |  3545 |

2）

用户

+------+---------+
|userid| name    |
+------+---------+
|  1   |   ale   |
|  2   |   ben   |
|  3   |   jak   |

我想实现：

piva

：

fieldid

=表中的1

customfields

+------+---------+-------+
|userid| fieldid | value |
+------+---------+-------+
|  1   |   1     |  5c54 |
|  2   |   1     |  cerc |
|  2   |   3     |  3545 |

+--------+---------+-------+
|name    | piva    | code  |
+--------+---------+-------+
|  ale   |   5c54  |       |
|  ben   |   cerc  |  3545 |

code

：

fieldid

=3在表

customfields

+------+---------+-------+
|userid| fieldid | value |
+------+---------+-------+
|  1   |   1     |  5c54 |
|  2   |   1     |  cerc |
|  2   |   3     |  3545 |

+--------+---------+-------+
|name    | piva    | code  |
+--------+---------+-------+
|  ale   |   5c54  |       |
|  ben   |   cerc  |  3545 |

我不知道如何实现这一点

您可以使用条件聚合：

select u.name,
       max(case when cf.fieldid = 1 then value end) as piva,
       max(case when cf.fieldid = 3 then value end) as code
from customfields cf join
     users u
     using (userid)
group by u.name;

---

这与

完全外部连接无关（MySQL不支持）。其中一种方法只是具有条件聚合的联接：

select u.name,
       max(case when cf.fieldid = 1 then value end) as piva,
       max(case when cf.fieldid = 3 then value end) as code
from customfields cf join
     users u
     using (userid)
group by u.name;

您可以尝试以下SQL查询：

Select u.name,
(Select value from customfields where fieldid = 1 and userid= u.userid) as piva, 
(Select value from customfields where fieldid =3 and userid = u.userid) as code 
from users u

这很好，但我很想了解发生了什么，你能帮我吗？@ciaoben使用
案例
表达式，我们可以为每个
fieldid
值生成一个值，即
fieldid=1
时的值和
fieldid=3
时的值
MAX
仅返回此值。