IFNULL（）仍然返回Null - MYSQL

时间：2015-06-26 08:17:24

标签： mysql

我正在编写一个查询，将一些日期组合在一起计算行数。我使用左连接将两个表连接在一起，第二列中的行是NULL我想输出0。

以下是我的数据片段。

交易表 

ITEM    DOE                  T_DOEDAY
42      11/06/2015 10:40    11/06/2015
14      12/06/2015 06:19    12/06/2015
2151    15/06/2015 07:27    15/06/2015
2114    16/06/2015 09:58    16/06/2015
98541   17/06/2015 07:15    17/06/2015
101     18/06/2015 10:14    18/06/2015
9854    19/06/2015 06:14    19/06/2015
2121    22/06/2015 07:14    22/06/2015
2354    23/06/2015 07:12    23/06/2015
252     24/06/2015 07:12    24/06/2015
5654    15/06/2015 10:12    25/06/2015
245     15/06/2015 09:11    26/06/2015
..............

属性表 

ITEM        DOE                 DOEDAY
CD      11/06/2015 10:40        11/06/2015
CD      12/06/2015 06:19        12/06/2015
OP      15/06/2015 07:27        15/06/2015
DF      16/06/2015 09:58        16/06/2015
CD      19/06/2015 06:14        19/06/2015
CD      22/06/2015 07:14        22/06/2015
OD      23/06/2015 07:12        23/06/2015
OD      24/06/2015 07:12        24/06/2015
OD      15/06/2015 10:12        25/06/2015
CD      15/06/2015 09:11        26/06/2015
...............

我在查询中尝试了以下内容 - 

SELECT * 
FROM (SELECT COUNT(DISTINCT T.ITEM) AS ITEM_COUNT, T.ITEM, T.DOE, T.DOEDAY AS T_DOEDAY, T.pos 
      FROM TRANSACTIONS T 
      group by T_DOEDAY ) J1 
LEFT JOIN 
    (SELECT IFNULL(COUNT(T1.DEFECT), 0) as DEFECT_COUNT, 
            T1.DOEDAY AS T1_DOEDAY 
     FROM ATTRIBUTES T1  
     GROUP BY T1_DOEDAY ) J2 
ON T_DOEDAY = T1_DOEDAY

其中输出以下内容 - 

ITEM_COUNT ITEM DOE                     T_DOEDAY  DEFECT_COUNT      T1_DOEDAY
32      10      11/06/2015 10:40        11/06/2015      0       11/06/2015
69      18      12/06/2015 06:19        12/06/2015      0       12/06/2015
49      2       15/06/2015 07:27        15/06/2015      0       15/06/2015
14      21      16/06/2015 09:58        16/06/2015      0       16/06/2015
3       6       17/06/2015 07:14        17/06/2015      NULL        NULL
16      18      18/06/2015 09:18        18/06/2015      NULL        NULL
31      22      19/06/2015 06:14        19/06/2015      0       19/06/2015
98      89      22/06/2015 07:14        22/06/2015      0       22/06/2015
14      204     23/06/2015 07:12        23/06/2015      0       23/06/2015
68      202     24/06/2015 07:12        24/06/2015      0       24/06/2015
26      8       15/06/2015 10:12        25/06/2015      0       25/06/2015
133     1       15/06/2015 09:11        26/06/2015      0       26/06/2015

所以我尝试了这个查询 - 

SELECT * 
FROM (SELECT COUNT(DISTINCT T.ITEM) AS ITEM_COUNT, T.ITEM, T.DOE, T.DOEDAY AS T_DOEDAY, T.pos 
      FROM TRANSACTIONS T group by T_DOEDAY ) J1 
LEFT JOIN 
    (SELECT IF(COUNT(T1.DEFECT) = NULL,0,COUNT(T1.DEFECT)) as DEFECT_COUNT, 
        T1.DOEDAY AS T1_DOEDAY 
     FROM ATTRIBUTES T1  
    GROUP BY T1_DOEDAY ) J2 
ON T_DOEDAY = T1_DOEDAY

再输出以下内容 - 

ITEM_COUNT ITEM DOE                     T_DOEDAY  DEFECT_COUNT      T1_DOEDAY
32      10      11/06/2015 10:40        11/06/2015      0       11/06/2015
69      18      12/06/2015 06:19        12/06/2015      0       12/06/2015
49      2       15/06/2015 07:27        15/06/2015      0       15/06/2015
14      21      16/06/2015 09:58        16/06/2015      0       16/06/2015
3       6       17/06/2015 07:14        17/06/2015      NULL        NULL
16      18      18/06/2015 09:18        18/06/2015      NULL        NULL
31      22      19/06/2015 06:14        19/06/2015      0       19/06/2015
98      89      22/06/2015 07:14        22/06/2015      0       22/06/2015
14      204     23/06/2015 07:12        23/06/2015      0       23/06/2015
68      202     24/06/2015 07:12        24/06/2015      0       24/06/2015
26      8       15/06/2015 10:12        25/06/2015      0       25/06/2015
133     1       15/06/2015 09:11        26/06/2015      0       26/06/2015

预期结果 

ITEM_COUNT ITEM DOE                     T_DOEDAY  DEFECT_COUNT      T1_DOEDAY
32      10      11/06/2015 10:40        11/06/2015      0       11/06/2015
69      18      12/06/2015 06:19        12/06/2015      0       12/06/2015
49      2       15/06/2015 07:27        15/06/2015      0       15/06/2015
14      21      16/06/2015 09:58        16/06/2015      0       16/06/2015
3       6       17/06/2015 07:14        17/06/2015      0        NULL
16      18      18/06/2015 09:18        18/06/2015      0        NULL
31      22      19/06/2015 06:14        19/06/2015      0       19/06/2015
98      89      22/06/2015 07:14        22/06/2015      0       22/06/2015
14      204     23/06/2015 07:12        23/06/2015      0       23/06/2015
68      202     24/06/2015 07:12        24/06/2015      0       24/06/2015
26      8       15/06/2015 10:12        25/06/2015      0       25/06/2015
133     1       15/06/2015 09:11        26/06/2015      0       26/06/2015

我不明白为什么IFNULL或IF函数不能正常工作，sql运行正常，没有错误被标记出来。

如果有人能对此有所了解，我将非常感激。

3 个答案:

答案 0 :(得分：0)

如果isnull为真，则必须添加默认值

SELECT * FROM (SELECT COUNT(DISTINCT T.ITEM) AS ITEM_COUNT, T.ITEM, T.DOE, T.DOEDAY AS T_DOEDAY, T.pos FROM TRANSACTIONS T group by T_DOEDAY ) J1 LEFT JOIN 
    (SELECT IFNULL(COUNT(T1.DEFECT),0) as DEFECT_COUNT, 
     T1.DOEDAY AS T1_DOEDAY FROM ATTRIBUTES T1  GROUP BY T1_DOEDAY ) J2 ON T_DOEDAY = T1_DOE

答案 1 :(得分：0)

我没有看到您使用IFNULL尝试以下查询。

SELECT * FROM (SELECT COUNT(DISTINCT T.ITEM) AS ITEM_COUNT, T.ITEM, T.DOE, T.DOEDAY AS T_DOEDAY, T.pos FROM TRANSACTIONS T group by T_DOEDAY ) J1 LEFT JOIN 
    (SELECT IFNULL(COUNT(T1.DEFECT),0) as DEFECT_COUNT, 
     T1.DOEDAY AS T1_DOEDAY FROM ATTRIBUTES T1  GROUP BY T1_DOEDAY ) J2 ON T_DOEDAY = T1_DOE

NULL Functions

答案 2 :(得分：0)

您需要在加入后执行IFNULL，而不是在进行计数的子查询中。当没有匹配的行时，LEFT JOIN会添加空值。

SELECT J1.*, IFNULL(defect_count, 0) AS defect_count
FROM (SELECT COUNT(DISTINCT T.ITEM) AS ITEM_COUNT, T.ITEM, T.DOE, T.DOEDAY AS T_DOEDAY, T.pos 
      FROM TRANSACTIONS T 
      group by T_DOEDAY ) J1 
LEFT JOIN
    (SELECT COUNT(*) as DEFECT_COUNT, 
            T1.DOEDAY AS T1_DOEDAY 
     FROM ATTRIBUTES T1  
     GROUP BY T1_DOEDAY ) J2 
ON J1.T_DOEDAY = J2.T1_DOEDAY
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