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DevJef's Mumbo-Jumbo «There's a bit of SQL in all of us»

If you work with SQL Server you will eventually encounter a JOIN statement. This JOIN statement can be stated as an ordinary JOIN, or as a sub select. In most cases I try to use JOINS (even though it’s just for readability), but in some cases it’s quicker to do it in a sub select (for example with production code, in which you don’t want to change the output but filter output).


Curious about the performance differences between the two, I tried to create a test. I created 2 tables with country information:










In this case I will JOIN both tables, based on DefaultAbbreviation. The queries I used look like this:


SELECT CA.UNAbbreviation
FROM TST_CountryAbbreviation CA
WHERE 1 = 1
AND CA.DefaultAbbreviation IN
    (SELECT DefaultAbbreviation
     FROM TST_CountryInformation)


SELECT CA.DefaultAbbreviation
FROM TST_CountryAbbreviation CA
INNER JOIN TST_CountryInformation CI
    ON CI.DefaultAbbreviation = CA.DefaultAbbreviation
WHERE 1 = 1


Both queries are executed, and the Execution Plans look like this:


Query 1:


And for query 2:


As you can see the Execution Plans are practically the same. But if you look at the Execution Plan if you execute both statements simultaneously:







So in this case it proves that a JOIN is slower than a sub select. But what if you want a specific value…? I used the same statements, except in both versions I ask for the specific “CountryProper” value “Netherlands”:





So the Execution Plans show that if you fetch a specific value, the costs of both queries are the same.


The conclusion is the same as always: check the Execution Plan if you’re not sure about the performance of your statements.






Posted on Tuesday, November 1, 2011 2:26 PM SQL Scripts , SQL Server | Back to top

Comments on this post: Join vs Sub Query

# re: Join vs Sub Query
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Smart to execute them simultaneously and look at how much resource of the batch they take.

And as always... have nice day
Left by michel on Nov 01, 2011 9:22 PM

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