MySQL - 查询 - 多个分组依据

Question

我有下表，其中我尝试对 type = 'printer' 进行求和，但是，我不想计算重复的 client_ids。所以我期望这样的结果：

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

一共有三行，其类型为打印机。 Sum 将它们全部相加，并返回 3。

如果您希望看到带有打印机的行的 1，否则看到 0，请尝试 < code>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

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

Answer 4

SELECT distinct client_id, 
     (IF(type = 'printer', 1,0)) 
FROM FOO 

（我猜：我不熟悉 IF(..)）