MySQL - 查询 - 多个组

Question

我有下表,其中我尝试SUM,如果type ='printer',但是,我不想重复client_ids.所以我希望这样的事情:

+------+-----------+-----------+
| k_id | client_id | type      |
+------+-----------+-----------+
|    1 |       100 | pc        | 
|    2 |       101 | printer   | 
|    3 |       101 | printer   | 
|    4 |       101 | printer   | 
|    5 |       102 | cellphone | 
+------+-----------+-----------+

查询:

  SELECT client_id, 
         SUM(IF(type = 'printer', 1,0)) 
    FROM FOO 
GROUP BY type, client_id;

结果:

+-----------+--------------------------------+
| client_id | SUM(IF(type = 'printer', 1,0)) |
+-----------+--------------------------------+
|       102 |                              0 | 
|       100 |                              0 | 
|       101 |                              3 | 
+-----------+--------------------------------+

预期结果:

+-----------+--------------------------------+
| client_id | SUM(IF(type = 'printer', 1,0)) |
+-----------+--------------------------------+
|       102 |                              0 | 
|       100 |                              0 | 
|       101 |                              1 | 
+-----------+--------------------------------+

Answer 1

使用:

   SELECT x.client_id,
          COUNT(DISTINCT y.type) 
     FROM FOO x
LEFT JOIN FOO y ON y.client_id = x.client_id
               AND y.type = 'printer'
 GROUP BY x.client

如果您不需要查看零计数的行:

   SELECT client_id, 
          COUNT(DISTINCT type)
     FROM FOO 
    WHERE type = 'printer'
 GROUP BY type, client_id;

Answer 2

有三种类型的行printer. Sum将它们全部添加,然后返回3.

如果您想查看1带打印机的行,或者0其他方式,请尝试max而不是sum:

MAX(IF(type = 'printer', 1,0))
^^^

编辑:要计算不同打印机的数量,您可以使用子查询:

SELECT  client_id
,       (
        select  count(*) 
        from    FOO as f2 
        where   f1.client_id = f2.client_id
                and type = 'Printer'
        )
FROM    FOO as f1
GROUP BY 
        client_id