Case when Statement affect the Performance.

Question

1- Do the Case when inside or outside the Where condition affect the performance , dose it affect the Query Plan of a Single Query , dose affect the Picking of the index to be used for a single Query.

2- is better to to Write 2 or more Queries each handling one option of the Case when value rather than writing a one Single Query having a case when statement.

Example:

 Select  Acc ,
 Case 
 when  Calculation_type  = 'M' then Min_value 
 when  Calculation_type  = 'E' then Ending_value
 Else 
 Average_BLC
 END
From Table 

alternative

Select Acc ,    Min_value 
From Table 
where   Calculation_type  = 'M'
union all 
Select  Acc , Ending_value        
From Table 
where  Calculation_type  = 'E'
union all 
Select   Acc , Average_BLC       
From Table 
where Calculation_type  = 'A'

Answer 1

In this case, the first option will almost certainly be more efficient since it only requires that you hit the table (and/or the indexes on the table) once rather than hitting them three times (I'm assuming that you intended the first query to have a WHERE calculation_type IN ('A', 'E', 'M') so that the results match the second query).

I suppose that if you have three different covering indexes, one for each of the queries you're UNION ALL -ing together and that it makes sense for the optimizer to use those indexes, there may be little difference.

Assuming you have to hit the table for some of these columns, however, you would want to be able to read the table row once and get all the column values rather than reading the block three different times for the three different subqueries.