没有重叠标准测试的T-SQL CASE语句

Question

Since the following code prints 'First' and 'Second' INSERT that order, can I conclude that the first condition satisfied is ALWAYS executed?

DECLARE @Constant1 int = 1
DECLARE @Constant2 int = 2

select 
     case 
          when @Constant1 = 1 
               then 'First'
          when @Constant1 = 1  and @Constant2 = 2
               then 'Second'               
     end as Result

select 
     case 
          when @Constant1 = 1  and @Constant2 = 2
               then 'Second'                    
          when @Constant1 = 1 
               then 'First'

     end as Result     

I know that sometimes parallel processing effects the outcome and I was trying to understand IF this type of situation that I see in Production would always return the same result.

This question is intended to understand if there is a potential issue in production code. If I were going to write the code anew, I think I would try to make the code explicitly mutually exclusive..

select 
     case 
          when @Constant1 = 1 and @Constant2 != 2
               then 'First'

          when @Constant1 = 1  and @Constant2 = 2
               then 'Second'                    

     end as Result     

Answer 1

The Documentation for CASE states.

Searched CASE expression:

• Evaluates, in the order specified, Boolean_expression for each WHEN clause.
• Returns result_expression of the first Boolean_expression that evaluates to TRUE.
• If no Boolean_expression evaluates to TRUE, the Database Engine returns the else_result_expression if an ELSE clause is specified, or a NULL value if no ELSE clause is specified.

So it will return the first true branch.

For a simple query such as in the question I would expect it to not evaluate the other branches either.

A few cases where this short circuiting behaviour does not work as expected/advertised are discussed in this DBA site question.

Does SQL Server read all of a COALESCE function even if the first argument is not NULL?

But just to be clear these issues do not affect the left to right precedence order of the result (except for the case when evaluating a later branch causes an error to occur such that no result is returned at all)