Nested Case Statement - Pulling the wrong value - sql

Question

Here is my syntax/question
,CASE WHEN nullif(ltrim(A),'') IS NOT NULL OR nullif(ltrim(B),'') IS NOT NULL THEN NULL
WHEN nullif(ltrim(C),'') IS NOT NULL THEN (
CASE WHEN nullif(MM,'') IS NOT NULL THEN MM
WHEN NN IS NOT NULL THEN XX
WHEN NN IS NULL THEN concat(UPPER(YY), ' ', UPPER(XX))
END
)
WHEN nullif(ltrim(D),'') IS NOT NULL OR nullif(ltrim(E),'') IS NOT NULL THEN concat(UPPER(XX), ' ', UPPER(YY))
ELSE ' '
END as 'Data_Item'
We have a series of statements to evaluate.
If fields A OR B are not null then we pull NULL
if field C is not null then we evaluate a nested case statement and this is where something is off with my code.
if field C is not null we should evaluate field MM if MM is not null we should return MM (this is what i should be getting but i'm not).
we continue to evaluate, if NN is not null we pull XX
if NN is null we concatenate YY space XX
and lastly if field D is not null or field E is not null then we concatenate fields XX space YY.
else we simply return a space
then end
--
In short, its a semi-complex series case statement, if the second scenario is true we have to evaluate like 3 or 4 nested statements. Till we find what is true.
for whatever reason my data is always returning the last statement (D is true) rather than the nested one.

Answer 1

This is your CASE simplified:
CASE
WHEN A <> '' OR B <> ''
THEN NULL
WHEN C <> ''
THEN CASE
WHEN MM <> '' THEN MM
WHEN NN IS NOT NULL THEN XX
WHEN NN IS NULL THEN Concat(Upper(YY), ' ', Upper(XX))
END
WHEN D <> '' OR E <> ''
THEN Concat(Upper(XX), ' ', Upper(YY))
ELSE ' '
END
Seems to match your logic ...