SQL Server：连接导致行太多

Question

I have two SQL Server tables.

The first table is called Content and contains -- among other things -- columns called

FileID, FileHighResolutionID, FileHighResolutionProID, FileVectorID

The second table is called Analytics and contains -- among other things -- a column called FileID . This column contains the value from one of the four aforementioned columns in Content .

Executing the following...

SELECT 
    * 
FROM 
    Analytics a
WHERE 
    a.Created BETWEEN '2017-03-07' AND '2017-03-08'

results in 782 rows.

But executing the following...

SELECT 
    * 
FROM 
    Analytics a
INNER JOIN 
    Content c ON (c.FileID = a.FileID OR c.FileHighResolutionID = a.FileID OR c.FileHighResolutionProID = a.FileID OR c.FileVectorID = a.FileID)
WHERE 
    a.Created BETWEEN '2017-03-07' AND '2017-03-08'

results in 843 rows.

I know I have something wrong with my JOIN, because now I have 61 too many records. I have tried INNER JOINs, LEFT OUTER JOINS, RIGHT OUTER JOINs, but each results in 61 mysterious extra records.

Can some SQL expert please review and tell me what I am doing wrong?

Answer 1

You must have figured out by now that the issue is that the OR is matching multiple columns in c . You get a separate row for each match. Voila! Unexpected rows.

One method to resolve this uses a "lateral join". This is like a correlated subquery, but it can return more than one column and more than one row (not needed here). In SQL Server, this is implemented using APPLY :

SELECT a.*, c.*
FROM Analytics a CROSS APPLY
     (SELECT TOP 1 c.*
      FROM Content c 
      WHERE a.FileID IN (c.FileId, c.FileHighResolutionID, c.FileHighResolutionProID, c.FileVectorID)
    ) c
WHERE a.Created BETWEEN '2017-03-07' AND '2017-03-08';

This returns an arbitrary matching row. You can get a specific row with an ORDER BY :

SELECT a.*, c.*
FROM Analytics a CROSS APPLY
     (SELECT TOP 1 c.*
      FROM Content c 
      WHERE a.FileID IN (c.FileId, c.FileHighResolutionID, c.FileHighResolutionProID, c.FileVectorID)
      ORDER BY (CASE a.FileId WHEN c.FileId THEN 1 c.FileHighResolutionID THEN 2 c.FileHighResolutionProID THEN 3 c.FileVectorID THEN 4
                END)
    ) c
WHERE a.Created BETWEEN '2017-03-07' AND '2017-03-08';

Note: I do agree with the answer that questions the use of BETWEEN with date/time values. This is dangerous, because times cause misleading logic. I strongly recommend one of the following:

WHERE a.Created = '2017-03-07'
WHERE a.Created >= '2017-03-07' AND a.Created < '2017-03-09';

Answer 2

If you don't need data from Content table, you could go for EXISTS :

SELECT *
FROM Analytics AS A
WHERE A.Created >= '2017-03-07'
    AND A.Created < '2017-03-08'
    AND EXISTS (
        SELECT *
        FROM Content AS C
        WHERE A.FileID IN (C.FileID, C.FileHighResolutionID, C.FileHighResolutionProID, C.FileVectorID)
        );

EXISTS will yield either true / false in WHERE condition and will not create dupes.

Answer 3

Another bad practice is using BETWEEN in a WHERE clause when filtering on dates

WHERE a.Created >= '20170307' AND a.Created < '20170308'

You have more rows in the second query because you are joining on multiple columns and this might be the reason why you are getting more records. In your first query you aren't joining with another table.