如何有SQL INNER JOIN接受空結果

Question

我有以下查詢：

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

將返回：

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

我想用返回的MD5，以獲取相應的客戶端名稱（FIRM）從該查詢，那麼行可能看起來像：

879100cf8aa8b993a8c53f0137a3a176 126122 Burger King 

所以我做了這個查詢：

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; 

這將返回類似：

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

其中一期工程，但我需要的，如果有一個給定的MD5沒有相應的公司來返回c.FIRM空值。例如：

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

我應該如何修改查詢仍然返回一行，即使沒有相應的c.FIRM？

Answer 1

與LEFT JOIN

Answer 2

內部聯接排除空值替換INNER JOIN;你想要一個LEFT OUTER連接。

Answer 3

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; 

當客戶表中的記錄不存在時，這會給你一個「未知」值。你顯然可以放棄那部分，只要返回c.FIRM，如果你想要實際的空值。

Answer 4

而不是做一個內部聯接，你應該做一個LEFT OUTER JOIN：

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

使用LEFT JOIN代替INNER JOIN

Answer 6

改變你的INNER JOIN到外連接.. 。

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;