LEFT JOIN multiple function

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Greetings,

If I LEFT JOIN several functions with a given query with each one of them having a join condition different from the other one. Will all those functions execute or the one with the correct condition only will be the one that is executed. I know that my words may be confusing so I will try to explain by an example

SQL

query LEFT JOIN function1(parameters) on categorycolumn = 1
      LEFT JOIN function2(parameters) on categorycolumn = 2

What I have tried:

Tried to google for an answer and trying to debug however the SSMS debugger is not working for me.

Posted 21-Aug-17 9:13am

Amr Mohammad Rashad

Updated 22-Aug-17 23:01pm

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RickZeeland · Answer 1 · 2017-08-21T09:47:00

Solution 1

Maybe you can use APPLY, see example here: sql server - How to make left outer join a UD-function with any sql table? - Stack Overflow[^]

Posted 21-Aug-17 9:47am

RickZeeland

Comments

Amr Mohammad Rashad 21-Aug-17 16:12pm

Both APPLY operator or the JOIN operator can be used and will have the same result but I was asking from the performance point of view that is from one side. From another side I was asking if there is a way preventing the execution of the query within the function instead of using IF statements within the function body to do such prevention.

Jörgen Andersson · Answer 2 · 2017-08-22T23:00:00

It's performance wise generally a bad idea to put functions in the FROM clause.
See if you can move them to the SELECT clause, then you can also filter them using a CASE.
Like this:

SQL

SELECT  CASE 
            WHEN categorycolumn = 1 THEN function1(parameters) 
            WHEN categorycolumn = 2 THEN function2(parameters)
        END AS MyResult
FROM...

If the results arent compatible it's easy enough to split into several CASEs.