Mutable logical condition in CASE expression 6


In our previous blog post Different data types in case expression, we have explored the behavior of CASE expression in case of different data types in “THEN” and “ELSE” part. Now in this post “Mutable logical condition in CASE expression”, we are going to unfold the behavior of CASE expression when we have a non deterministic function in logical expression (WHEN part).

Mutable logical condition in CASE expression

Have a look on the below query:

--Query 1
SELECT
CASE ABS(CHECKSUM(NEWID())) % 3
WHEN 0 THEN 'Case 0'
WHEN 1 THEN 'Case 1'
WHEN 2 THEN 'Case 2'
END

 

In above query, logical expression “ABS(CHECKSUM(NEWID())) % 3” of CASE expression, has a non deterministic function NEWID() wrapped inside a CHECKSUM function to generate some random number on fly. Going further the CHECKSUM value has also wrapped inside a ABS function which insures a positive number. Finally there is a Mod operator with value 3 which narrows down the output result to 0 or 1 or 2 only. To catch this output, inside the WHEN part of CASE expression, we have three switches 0, 1 and 2 respectively to return some demo value.

If you execute the above code repeatedly for 6 – 7 times (may vary a bit), you may get unexpected “NULL” value in the output. As we have all possible conditions in WHEN part to catch the output of logical condition, why we are getting this unexpected NULL value? Its just because of the nature of execution of CASE expression. Above query (Query 1) is equivalent to the below query (Query 2):

--Query 2
SELECT
CASE WHEN ABS(CHECKSUM(NEWID())) % 3 = 0 THEN 'Case 0'
WHEN ABS(CHECKSUM(NEWID())) % 3 = 1 THEN 'Case 1'
WHEN ABS(CHECKSUM(NEWID())) % 3 = 2 THEN 'Case 2'
END

Logical condition inside CASE expression executes for each WHEN part. Even we have used the logical expression “ABS(CHECKSUM(NEWID())) % 3” once in Query 1, it will execute for each WHEN part (3 times in our case).

Why NULL?

When first WHEN clause (WHEN 0) calls the logical condition, it might have got 1 or 2, when second WHEN clause (WHEN 1) calls, it might have got 0 or 2. And finally when third WHEN clause (WHEN 2) calls, it might have got 0 or 1 and all WHEN parts fails to catch. CASE expression always has an ELSE part even we don’t put it explicitly there and here we are getting the NULL from this ELSE part.

 

Switch Case
Result received from logical conditionAction
WHEN 0Got 1 or 2By passed
WHEN 1Got 0 or 2By passed
WHEN 2Got 0 or 1By passed
ELSEAlways there even not written explicitlyCaught and returned NULL

 

Handle mutable logical condition in CASE expression

To handle such kind of situations, simply use derived table to fix the output of logical condition first, then from there, we can create switch cases to catch the output as below:

--Query 3
SELECT
CASE WHEN DT.Col = 0 THEN 'Case 0'
WHEN DT.Col = 1 THEN 'Case 1'
WHEN DT.Col = 2THEN 'Case 2'
END
FROM
(SELECT ABS(CHECKSUM(NEWID())) % 3 AS Col) DT

 

In above query (Query 3), we have fixed the output in a derived table first and then written switch cases from there. In case of Query 3 it will never output the unexpected NULL value.

I hope you have enjoyed this post and thanks for your reading. I would also like to ask you to share your method in comment section to handle this scenario.

Do post your comments and suggestions, rate this post and most important, please keep sharing.

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Gopal Krishna Ranjan

About Gopal Krishna Ranjan

I am Gopal Krishna Ranjan, having 8 years of industry experience in Software development. I have a head down experience in Database, Data Warehouse, Big Data and cloud technologies and have implemented end to end Database, Data Warehouse,  Big Data and Cloud Solutions.
I have extensively worked on SQL Server, Python, Hadoop, Hive, Spark, Azure, Machine Learning, and MSBI (SSAS, SSIS, and SSRS). I also have good experience in windows and web application development using ASP.Net and C#.


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6 thoughts on “Mutable logical condition in CASE expression

  • Ananthram

    A very informative article indeed! Just out of curiosity, do you have a particular reason to have used a derived table in this case to work around the unexpected NULL values? I would think a declaring a variable and then using it instead of a derived table would work just fine.
    Something like this:

    DECLARE @I TINYINT = ABS(CHECKSUM(NEWID())) % 3;
    SELECT
    CASE ABS(CHECKSUM(NEWID())) % 3
    WHEN 0 THEN ‘Case 0’
    WHEN 1 THEN ‘Case 1’
    WHEN 2 THEN ‘Case 2’
    END

    Good article nontheless.

    • Gopal Krishna Ranjan
      Gopal Krishna Ranjan Post author

      Hi Ananthram,
      Of course you can use a variable in such scenarios where you have to work with one column and some trivial queries. But in case you have a complex query with multiple columns where data is coming from multiple tables, derived tables, views, functions etc by joining them and you have this variable CASE condition in between somewhere. It might not be possible to deal with variables. And in that case, we can use derived tables as above.
      Thanks for your question and suggestion too.

    • Gopal Krishna Ranjan
      Gopal Krishna Ranjan Post author

      Hi Arun,
      I am little bit confuse about your question. What i could guess at my best, it seems that you have a concern about the performance of hidden ELSE part and explicitly mentioned ELSE part in CASE expression.
      As best of my knowledge and as I have already mentioned in my post, there is always an ELSE part either you mention it or not. So it should not have any performance impact on the query.

  • Gerald Britton

    An interesting and informative article! Thanks!! I must say though that you use “mutable” in a novel fashion. Usually in CS, mutability is a property of an object that denotes that it can be modified (or not) after creation. The discussion often concerns functional languages. In F# for example, all objects are immutable by default, though they can be marked mutable — often needed for inter-operation with other languages. The usage in this article does not concern an object in that sense of course. That is,

    ABS(CHECKSUM(NEWID())) % 3

    does not create an object per se. “non-deterministic” or even just “variable” might have been my choice.