SQL - 如何有条件地调用查询的一部分

Question

I need to write a SQL query like this:

do step1

do step2

do step3 - this step does a lot of stuffs: define variables, use some variables defined in step 1, query a lot of data into temporary table...

if(cond1)

   if(cond2)        
 
       Begin            
          do step 4             
          call step 3 here      
       End  
    else
    
      Begin             
           do step 5        
      End  
  else  
 
  Begin         
     call step 3 here   
  End

How to make step 3 a reusable query to avoid calling step 3 unnecessarily? notice that step 3 should be a part of the whole query

When I try to create a @step3Query NVARCHAR(MAX), SET @step3Query = '...' Then call this query in appropriated places by call "EXEC sp_executesql @step3Query". But I got a lot of errors, and I am not sure that is a correct way to do task like that. Any suggestions would be highly appreciated.

Answer 1

Here is one method that could work (depending on your exact circumstance). The main thing for this is that it's easy, as well as easy to read.

You could have 3 variables to use as flags eg, @Query3_Flag, @Query4_flag, @Query5_flag.

Your conditional checks just set these flags eg,

IF (condition1)
    BEGIN
    SET @Query4_Flag = 1;
    SET @Query3_flag = 1;
    SET @Query5_flag = 0;
    END

Before each of queries 3, 4 and 5, have the IF statement check the flag and only run the query if the flag is set to 1 eg,

IF @Query3_Flag = 1 
    BEGIN
    (run Query 3)
    END

Note that the queries will need to be in the correct order.