Question

如果我运行SELECT查询并返回其中包含冗余信息的行，那么如何在一行中显示该信息？

当前我的查询是这样的：

SELECT email, first_name, group FROM membership

然后返回结果集，如下所示：

------------------------------------
|    email    | first_name | group |
------------------------------------
|  bob@co.uk  |     bob    |   g1  |
|  jim@co.uk  |     jim    |   g1  |
|  jim@co.uk  |     jim    |   g2  |
|  jane@co.uk |    henry   |   g2  |
------------------------------------

相反，我需要结果集如下所示：

--------------------------------------------
|    email    | first_name | group | group |
--------------------------------------------
|  bob@co.uk  |     bob    |   g1  |       |
|  jim@co.uk  |     jim    |   g1  |   g2  |
|  jane@co.uk |    henry   |   g2  |       |
--------------------------------------------

重要的是，尽管有多余的名称，但列名“ group”的名称不要不同。

编辑：为清楚起见，组数可能会更改。 edit2：我用于初始查询的完整代码是：

SELECT 
    persons.mail AS email,
    persons.fname AS 'first_name',
persons.lname AS 'last_name',
groups.name as add_list
FROM group_membership
LEFT JOIN groups ON group_membership.gid = groups.gid
LEFT JOIN persons ON group_membership.uid = persons.uid
WHERE persons.mail != ""
ORDER BY last_name

Answer 1

您要的是不可能的。如果有四行，其值：

|  jim@co.uk  |     jim    |   g2  |

您希望结果中有4列名为group的列，而不是当前显示的group的两列吗？

您可能想要的是：

select email, first_name, group_concat(`group`) group by email, first_name

Answer 2

对于您的示例数据，它将这样做：

.java

请参见demo。
结果：

select email, first_name,
  min(`group`) `group`, 
  case when max(`group`) <> min(`group`) then max(`group`) end `group`
from membership
group by email, first_name

Answer 3

根据上述示例，以下内容将适用

     SELECT email, first_name, max(case 
        when 
      group ='g1' then group else null end) 
       as groupg1, 
      max(case 
        when 
      group ='g2' then group else null end) 
       as groupg2
      FROM membership group by email, 
      first_name

如何根据返回的行选择多列？

3 个答案: