MySQL -查询-多个分组依据

Question

我有下面的表格，我正在尝试计算if type =‘client_ids’的总和，但是，我不想计算重复类型。所以我希望是这样的：

+------+-----------+-----------+
| 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

其中有三行的类型为printer。Sum将它们相加，然后返回3。

如果您希望为具有打印机的行显示1，否则为0，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，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

Answer 2

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

如果您不需要查看计数为零的行：

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

Answer 3

SELECT client_id, if( `type` = 'printer', 1, 0 )
FROM foo
GROUP BY TYPE , client_id