mysql - 如何根据返回的行选择多列？

Question

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

目前我的查询是这样的:

SELECT email, first_name, group FROM membership

返回的结果集如下:

------------------------------------
|    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，列名称“group”尽管是多余的，但不能以不同的方式命名。

编辑:为清楚起见，组数可能会发生变化。 edit2:我在初始查询中使用的完整代码是:

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

你要求的是不可能的。如果有四行的值为:

|  jim@co.uk  |     jim    |   g2  |

您是否希望结果有 4 个名为 group 的列，而不是您当前显示的 group 的两列？

你可能想要的是:

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