How to have SQL INNER JOIN accept null results - sql

Question

I have the following query:
SELECT TOP 25 CLIENT_ID_MD5, COUNT(CLIENT_ID_MD5) TOTAL
FROM dbo.amazonlogs
GROUP BY CLIENT_ID_MD5
ORDER BY COUNT(*) DESC;
Which returns:
283fe255cbc25c804eb0c05f84ee5d52 864458
879100cf8aa8b993a8c53f0137a3a176 126122
06c181de7f35ee039fec84579e82883d 88719
69ffb6c6fd5f52de0d5535ce56286671 68863
703441aa63c0ac1f39fe9e4a4cc8239a 47434
3fd023e7b2047e78c6742e2fc5b66fce 45350
a8b72ca65ba2440e8e4028a832ec2160 39524
...
I want to retrieve the corresponding client name (FIRM) using the returned MD5 from this query, so a row might look like:
879100cf8aa8b993a8c53f0137a3a176 126122 Burger King
So I made this query:
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;
This returns something like:
879100cf8aa8b993a8c53f0137a3a176 126122 Burger King
06c181de7f35ee039fec84579e82883d 88719 McDonalds
703441aa63c0ac1f39fe9e4a4cc8239a 47434 Wendy's
3fd023e7b2047e78c6742e2fc5b66fce 45350 Tim Horton's
Which works, except I need to return an empty value for c.FIRM if there is no corresponding FIRM for a given MD5. For example:
879100cf8aa8b993a8c53f0137a3a176 126122 Burger King
06c181de7f35ee039fec84579e82883d 88719 McDonalds
69ffb6c6fd5f52de0d5535ce56286671 68863
703441aa63c0ac1f39fe9e4a4cc8239a 47434 Wendy's
3fd023e7b2047e78c6742e2fc5b66fce 45350 Tim Horton's
How should I modify the query to still return a row even if there is no corresponding c.FIRM?

Answer 1

Replace INNER JOIN with LEFT JOIN

Answer 2

use LEFT JOIN instead of INNER JOIN

Answer 3

Instead of doing an INNER join, you should do a 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 4

An inner join excludes NULLs; you want a LEFT OUTER join.

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;
This will give you a value of "Unknown" when records in the customers table don't exist. You could obviously drop that part and just return c.FIRM if you want to have actual nulls instead.

Answer 6

Change your INNER JOIN to an 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;