GROUP BY with nested case expression - is there better way? - sql

Question

SQL server 2012
I am taking fees received and multiplying them by different factors based on how long the Client has been a Client. The group by clause is fairly straight forward. However, my select gets awkward when I want to use this criteria in different ways:
select mp.professionals
,case when sl.stmndate < dateadd(year, 3, m.qClientOpenDate) then 'New' else 'Old' end age -- straight forward
,case (case when sl.stmndate < dateadd(year, 3, m.qClientOpenDate) then 'New' else 'Old' end) -- nested case
when 'New' then sum(fees) * 0.5
when 'Old' then sum(fees) * 0.25
else 0
end Credit
,case (case when sl.stmndate < dateadd(year, 3, m.qClientOpenDate) then 'New' else 'Old' end) -- nested case
when 'New' then 'Welcome!'
when 'Old' then 'Thank you for being a long-time Client!'
end Greeting
from mattersprofessionals mp
inner join matters m on m.matters = mp.matters
inner join stmnledger sl on sl.matters = mp.matters
group by mp.professionals, case when sl.stmndate < dateadd(year, 3, m.qClientOpenDate) then 'New' else 'Old' end
I suppose I should mention this is a simplified version of my actual case statements.
I was hoping there was a way to group by sl.stmndate < dateadd(year, 3, m.qClientOpenDate) as a boolean or something so I don't have to do the nested case expressions.
I know I could do a sub-query on the basic case and then do more case expressions in the outer query. But that's just rearranging the same nested case concept.

Answer 1

When the values are calculated directly from the row I tend to use cross apply for this as it is more concise than adding a derived table/CTE whose only purpose is to define a column alias but still needs to project out the remaining columns and contain a FROM.
This is not an option for expressions that reference window functions or aggregate functions but will work fine here.
select mp.professionals
,ca.age -- straight forward
,case ca.age -- nested case
when 'New' then sum(fees) * 0.5
when 'Old' then sum(fees) * 0.25
else 0
end Credit
,case ca.age -- nested case
when 'New' then 'Welcome!'
when 'Old' then 'Thank you for being a long-time Client!'
end Greeting
from mattersprofessionals mp
inner join matters m on m.matters = mp.matters
inner join stmnledger sl on sl.matters = mp.matters
cross apply (select case when sl.stmndate < dateadd(year, 3, m.qClientOpenDate) then 'New' else 'Old' end) ca(age)
group by mp.professionals, ca.age

Answer 2

You can simplify the query a little by pre-computing the case expression in a subquery:
select
professionals,
age,
sum(fees) * case age
when 'New' then 0.5
when 'Old' then 0.25
else 0
end credit,
case age
when 'New' then 'Welcome'
when 'Old' then 'Thank you for being a long-time Client!'
end greeting
from (
select
mp.professionals,
case when sl.stmndate < dateadd(year, 3, m.qClientOpenDate)
then 'New'
else 'Old'
end age
from mattersprofessionals mp
inner join matters m on m.matters = mp.matters
inner join stmnledger sl on sl.matters = mp.matters
) t
group by professionals, age
Note: the else branch in the fees calculation is probably never reached (since age always takes either value 'New' or 'Old').

Answer 3

Technically I think the answer to my question is "No". There isn't a way to group by the boolean portion of a case statement so it can be used in various ways within the select.
I upvoted GMB's and Martin Smith's answers as they are helpful and informative. I am playing with the cross apply, a learning experience for sure.