我怎样才能用 MySQL 做到这一点？

Question

我使用的是 MySQL，有两个数据库表如下：

用户

id   username
--------------
1    Bill
2    Steve

对象

user_id   key   value
----------------------
1         A     X
1         B     Y
1         C     Z
2         A     S
2         C     T

产生以下结果需要什么查询？

username   A  B  C
-------------------
Bill       X  Y  Z
Steve      S     T

我已经用 INNER JOIN 尝试过这个，但最终有 5 行（每个对应的对象行一个）。

非常感谢任何帮助。

Answer 1

如果事先知道'A'、'B' 和'C'，您可以这样做：

SELECT users.username,
       ( SELECT objects.value
           FROM objects
          WHERE objects.user_id = users.id
            AND objects.`key` = 'A'
       ) AS a,
       ( SELECT objects.value
           FROM objects
          WHERE objects.user_id = users.id
            AND objects.`key` = 'B'
       ) AS b,
       ( SELECT objects.value
           FROM objects
          WHERE objects.user_id = users.id
            AND objects.`key` = 'C'
       ) AS c
  FROM users
 ORDER
    BY users.username
;

Answer 2

select u.username
     , oA.value A
     , oB.value B
     , oC.value C
  from users u
left
  join objects oA
    on u.id = oA.user_id
   and oA.key = 'A'
left
  join objects oB
    on u.id = oB.user_id
   and oB.key = 'B'
left
  join objects oC
    on u.id = oC.user_id
   and oC.key = 'C'

Answer 3

也许这不是您要的查询，但这是我在您的情况下使用过的简洁明了的查询：

select objects.*, matrix.*
from 
        (select users.id, o.key
          from users, (select distinct key from objects) as o
        ) 
as matrix left join objects on matrix.id = objects.user_id
                           and matrix.key = objets.key
order by matrix.id, matrix.key

此查询“填充”空白。因此，您可以使用两个嵌套的 foreach（或任何类似的）来使用结果集并绘制所需的表格。