Why would I get a difference in using a inner join vs intersect?

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Why would I get a difference in using a inner join vs intersect?

itmasterw 60042

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Points: 2852
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May 21, 2018 at 10:41 am

#335903

Hi,
I have two tables that I want to see what records are in both tables. So, I used an intersect and my boss looked at the results and did not feel that it was right. He used an inner join and got a different number. I always thought that they did the same thing and that you could use either and get the same results.
Does anyone know why we would get different results?
Below is the two methods that we used Name in the one table holds the first and the last name and that is why it is separated out.
--Here it returns 590
SELECT DISTINCT                 LEFT(CAST(al.Name AS VARCHAR(25)), CASE WHEN charindex(' ', CAST(al.Name AS VARCHAR(25))) = 0 THEN                 LEN(CAST(al.Name AS VARCHAR(25))) ELSE charindex(' ', CAST(al.Name AS VARCHAR(25))) - 1 END) as FristName,                 RIGHT(al.Name, CASE WHEN charindex(' ', al.Name) = 0 THEN                 LEN(CAST(al.Name AS VARCHAR(25))) ELSE charindex(' ', CAST(al.Name AS VARCHAR(25))) - 1 END) as LastNmae,                 Right(al.[SSN],4) AS SSN   FROM            [dbo].[AllLoans] al
intersect
SELECT DISTINCT
                [first_name],
                [last_name],
                RIGHT([ssn_no],4)
FROM            [DbInfo]
--Here I get 16534
SELECT DISTINCT                 LEFT(CAST(al.Name AS VARCHAR(25)), CASE WHEN charindex(' ', CAST(al.Name AS VARCHAR(25))) = 0 THEN                 LEN(CAST(al.Name AS VARCHAR(25))) ELSE charindex(' ', CAST(al.Name AS VARCHAR(25))) - 1 END) as FristName,                 RIGHT(al.Name, CASE WHEN charindex(' ', al.Name) = 0 THEN                 LEN(CAST(al.Name AS VARCHAR(25))) ELSE charindex(' ', CAST(al.Name AS VARCHAR(25))) - 1 END) as LastNmae,                            Right(al.[SSN],4) AS SSN   FROM            [dbo].[AllLoans] al INNER JOIN [DbInfo] di                 ON Right(al.[SSN],4) = RIGHT(cast(di.ssn_no as nvarchar(9)), 4)

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Phil Parkin SSC Guru Points: 245813 More actions · Answer 1

Your INTERSECT query is comparing all of the columns whereas the JOIN version compares only right(SSN,4) ... explaining why the second returns more matches.

If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.

sgmunson SSC Guru Points: 110588 More actions · Answer 2

Basically, matching only on the last 4 isn't going to give you a correct result, as two people can easily have the last 4 of their SSN be the same. That kind of matching is going to generate what amounts to tying records from one person to records of some other person. Bad idea.

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Jeff Moden SSC Guru Points: 1000539 More actions · Answer 3

Also, INTERSECT matches on NULL = NULL whereas Joins usually do not (default server setting).

--Jeff Moden

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First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.
"Change is inevitable... change for the better is not".

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itmasterw 60042 SSCrazy Points: 2852 More actions · Answer 4

itmasterw 60042

SSCrazy

Points: 2852

May 22, 2018 at 10:02 am

#1991452

Thanks guys this all really helps

Matt Miller (4) SSC Guru Points: 124210 More actions · Answer 5

itmasterw 60042 - Tuesday, May 22, 2018 10:02 AM
Thanks guys this all really helps

one last item - from MS docs:

INTERSECT returns distinct rows that are output by both the left and right input queries operator.

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