Question

我有以下查询：

SELECT TOP 25 CLIENT_ID_MD5, COUNT(CLIENT_ID_MD5) TOTAL 
FROM dbo.amazonlogs 
GROUP BY CLIENT_ID_MD5 
ORDER BY COUNT(*) DESC;

返回：

283fe255cbc25c804eb0c05f84ee5d52    864458
879100cf8aa8b993a8c53f0137a3a176    126122
06c181de7f35ee039fec84579e82883d    88719
69ffb6c6fd5f52de0d5535ce56286671    68863
703441aa63c0ac1f39fe9e4a4cc8239a    47434
3fd023e7b2047e78c6742e2fc5b66fce    45350
a8b72ca65ba2440e8e4028a832ec2160    39524
...

我想使用此查询中返回的MD5检索相应的客户端名称（FIRM），因此行可能如下所示：

879100cf8aa8b993a8c53f0137a3a176    126122    Burger King

所以我提出了这个问题：

SELECT a.CLIENT_ID_MD5, COUNT(a.CLIENT_ID_MD5) TOTAL, c.FIRM 
FROM dbo.amazonlogs a 
  INNER JOIN dbo.customers c 
    ON c.CLIENT_ID_MD5 = a.CLIENT_ID_MD5
GROUP BY a.CLIENT_ID_MD5, c.FIRM 
ORDER BY COUNT(*) DESC;

返回类似于：

的内容

879100cf8aa8b993a8c53f0137a3a176    126122    Burger King
06c181de7f35ee039fec84579e82883d    88719     McDonalds
703441aa63c0ac1f39fe9e4a4cc8239a    47434     Wendy's
3fd023e7b2047e78c6742e2fc5b66fce    45350     Tim Horton's

哪个有效，但如果给定的MD5没有相应的FIRM，我需要为c.FIRM返回一个空值。例如：

879100cf8aa8b993a8c53f0137a3a176    126122    Burger King
06c181de7f35ee039fec84579e82883d    88719     McDonalds
69ffb6c6fd5f52de0d5535ce56286671    68863
703441aa63c0ac1f39fe9e4a4cc8239a    47434     Wendy's
3fd023e7b2047e78c6742e2fc5b66fce    45350     Tim Horton's

即使没有相应的c.FIRM，如何修改查询仍然返回一行？

Answer 1

将INNER JOIN替换为LEFT JOIN

Answer 2

使用LEFT JOIN代替INNER JOIN

Answer 3

内连接排除NULL;你想要一个LEFT OUTER加入。

Answer 4

您应该进行LEFT OUTER加入，而不是进行INNER连接：

SELECT 
    a.CLIENT_ID_MD5, 
    COUNT(a.CLIENT_ID_MD5) TOTAL, 
    ISNULL(c.FIRM,'') 
FROM 
    dbo.amazonlogs a LEFT OUTER JOIN 
    dbo.customers c ON c.CLIENT_ID_MD5 = a.CLIENT_ID_MD5
GROUP BY 
    a.CLIENT_ID_MD5, 
    c.FIRM 
ORDER BY COUNT(0) DESC

http://www.w3schools.com/sql/sql_join.asp

Answer 5

SELECT a.CLIENT_ID_MD5, COUNT(a.CLIENT_ID_MD5) TOTAL, IsNull(c.FIRM, 'Unknown') as Firm
FROM dbo.amazonlogs a 
    LEFT JOIN dbo.customers c ON c.CLIENT_ID_MD5 = a.CLIENT_ID_MD5 
GROUP BY a.CLIENT_ID_MD5, c.FIRM ORDER BY COUNT(*) DESC;

当customers表中的记录不存在时，这将为您提供值“Unknown”。如果你想要实际的空值，你显然可以删除那部分而只返回c.FIRM。

Answer 6

将您的INNER JOIN更改为OUTER JOIN ...

SELECT a.CLIENT_ID_MD5, COUNT(a.CLIENT_ID_MD5) TOTAL, c.FIRM
FROM dbo.amazonlogs a
LEFT OUTER JOIN dbo.customers c
  ON c.CLIENT_ID_MD5 = a.CLIENT_ID_MD5
GROUP BY a.CLIENT_ID_MD5, c.FIRM
ORDER BY COUNT(*) DESC;

Answer 7

WITH amazonlogs_Tallies
     AS
     (
      SELECT a.CLIENT_ID_MD5, COUNT(a.CLIENT_ID_MD5) TOTAL 
        FROM dbo.amazonlogs a 
       GROUP 
          BY a.CLIENT_ID_MD5 
     ), 
     amazonlogs_Tallies_Firms
     AS
     (
      SELECT a.CLIENT_ID_MD5, a.TOTAL, c.FIRM 
        FROM amazonlogs_Tallies a 
             INNER JOIN dbo.customers c 
                ON c.CLIENT_ID_MD5 = a.CLIENT_ID_MD5
     )
SELECT CLIENT_ID_MD5, TOTAL, FIRM
  FROM amazonlogs_Tallies_Firms
UNION 
SELECT CLIENT_ID_MD5, TOTAL, '{{NOT_KNOWN}}'
  FROM amazonlogs_Tallies
EXCEPT
SELECT CLIENT_ID_MD5, TOTAL, '{{NOT_KNOWN}}'
  FROM amazonlogs_Tallies_Firms;

如何让SQL INNER JOIN接受null结果

7 个答案: