Question

I have a question about sub-queries and case statements

I have two case statements in the same query:

• One has a sub-query that is used to determine if a column has a match.
• I’d like the other to check if theres a match [among other checks], then tag a value.
• However, t-SQL will not let me reference my first field (generated from the case statement) within my second case statement.
• This forces me to add the subquery into my second case statement and do away with the first case statement
• When I do this, my query goes from 13 seconds to 2.5 minutes
• When I remove the subquery altogether from my query, it takes 8 seconds to run

Question 1: Can case-statement-generated fields be referenced in subsequent case statements in the same query?

Question 2: Why does my query take only 5 seconds longer when I have the subquery in an isolated case statement but 2 minutes longer when that subquery is in a case statement that has 4-5 other checks?

1st Case Statement

CASE WHEN (SELECT xxx.xxx from xxx) THEN 'Y' 
     END AS "Match_Ind",

Second Case Statement

CASE WHEN condition 1 = true THEN 'cond1'
    WHEN condition 2 = true THEN 'cond2'
    WHEN Match_Ind = 'Y' THEN 'matched'
    END AS "Match Detail"

Answer 1

You should consider posting your full query but if you want to reference the result of the first CASE inside of another CASE statement, then you can wrap it in a SELECT similar to this:

select 
    CASE 
        WHEN condition 1 = true THEN 'cond1'
        WHEN condition 2 = true THEN 'cond2'
        WHEN Match_Ind = 'Y' THEN 'matched'
    END AS Match Detail
from 
(
    SELECT CASE 
        WHEN conditionHere -- (SELECT xxx.xxx from xxx)
        THEN 'Y' 
        END AS Match_Ind,
        othercols
    from yourtable
) x