I’m commonly asked whether whether I can have a CASE Statement in the WHERE Clause. There are a number of examples using the CASE WHEN construct in SQL, such as the SELECT columns or in ORDER BY clauses, but we tend to forget CASE can be used wherever an expression is expected.

Where Can I use a CASE Statement?

You can use a CASE Statement anywhere a valid expression is used within the SELECT statement such as the WHERE clause’s filter criteria.

Example Query

Suppose we want to get all people from the Persons table whose persontype is either VC or IN. To do this with CASE you could write:

SELECT FirstName, LastName, PersonType
FROM Person.Person
WHERE 1 = CASE
WHEN PersonType = 'VC' THEN 1
WHEN PersonType = 'IN' THEN 1
ELSE 0
END

The idea here is to test PersonType for either VC or IN. If it matches the corresponding CASE WHEN return a 1. The filter matches and the row is included in the result.

OK, so now you can see that you can use CASE statement within a WHERE clause. Does that mean you should?

Personally I think it comes down to readability. In this case, the logic is pretty simple. Here I think it makes sense to stick with the basics. Just use Boolean OR or the IN operator.

Here is the example with OR

SELECT FirstName, LastName, PersonType
FROM Person.Person
WHERE PersonType = 'VC' or PersonType = 'IN'

Here is the same example with SQL IN

SELECT FirstName, LastName, PersonType
FROM   Person.Person
WHERE  PersonType in ('VC','IN')

Which one one would you use?

In this case I would tend to use the IN operator, it is succinct.

That said, don’t discount CASE! I find it extremely useful when working with more complex logic. In these situation, if you format the statement correctly, the code is much easier to read than nested Boolean operators, and it something you just can’t do with IN alone.

About the author 

Kris Wenzel

Kris Wenzel has been working with databases over the past 30 years as a developer, analyst, and DBA. He has a BSE in Computer Engineering from the University of Michigan and a MBA from the University of Notre Dame. Kris has written hundreds of blog articles and many online courses. He loves helping others learn SQL.

  • Hi Kris,

    You answered my initial question as to why would I use a CASE in your example as I’d of generally used the IN, so could you provide an example of where you would definitely prefer to use a CASE?

    Regards
    Andy

    • Kris,

      Up to now, I had only used CASE in my select clause. I just developed a query, though, which checked the timestamp in order to deliver the results of same day versus previous day. The reason for the need is that we run a 2 shift operation and 2nd shift runs until 1AM. We wanted a query where the user would not have to change anything, and would instead just run the query as is to get their day’s results. By using CASE in the WHERE clause, we were able to accomplish this.

  • I used it for a report query when a field was returning ‘Y’ or null and I was comparing it to a boolean parameter:

    WHERE @OnPublishedProgram = CASE
    WHEN On_Published_Program = ‘Y’ THEN 1
    ELSE 0
    END

    • Hi, The optimizer will try to use an index. In the simple case I provided it verified it did use a clustered index scan, but I imagine as your CASE gets more complicated there’s less a chance, especially one with several columns to evaluate within the expressions.

  • I found this post while trying to figure out the correct syntax for CASE WHEN in a WHERE clause and it helped – thank you.

    >> OK, so now you can see that you can use CASE statement within a WHERE clause. Does that mean you should?

    I have a Table Function which is split into 4 sections with IF | ELSE statements where each section contains a variant of the same query comprising SELECT, FROM, WHERE, GROUP BY, ORDER BY & OFFSET FETCH and where each differs only by the conditions in the WHERE clause. This is to allow the client to define the extent of the data that’s returned, ie: {booked events only, non-booked events only or all} and {future events only, past events only or all}.

    1. WHERE [TFP].[Projects].[UserID] IN (SELECT [UserID] FROM @TempTable_ColleagueUserIDs) AND [TFP].[ProposalRevs].[Booked] > 0 AND [TFP].[Projects].[EventDate] >= CAST(GETDATE() AS DATE)

    2. WHERE [TFP].[Projects].[UserID] IN (SELECT [UserID] FROM @TempTable_ColleagueUserIDs) AND [TFP].[ProposalRevs].Booked > 0

    3. WHERE [TFP].[Projects].[UserID] IN (SELECT [UserID] FROM @TempTable_ColleagueUserIDs) AND [TFP].[Projects].EventDate >= CAST(GETDATE() AS DATE)

    4. WHERE [TFP].[Projects].[UserID] IN (SELECT [UserID] FROM @TempTable_ColleagueUserIDs)

    I could greatly reduce the length of the overall query by removing the IF | ELSE blocks and inserting CASE WHEN into the WHERE clause. It would also mean that any common change I need to make could be done in just 1 query rather than 4.

    I’m curious if there’s any reason I shouldn’t do this but, more than that, validation as to whether having a single Table Function return data in 4 different ways is OK, ie: good practice, or if it’s perhaps better practice to have 4 distinct Table Functions.

    Thank you for any insight.

    Jason.

  • this was perfect…thank you thank you. Had a bunch of nested data with left joins and needed an easy way to eliminate rows where a specific value was null

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