Combining groups in GROUP BY - sql

Question

I have a query that groups by the field description. (There are 9 total descriptions.) I need to somehow combine 3 of these descriptions into one group, giving me a total of 7 descriptions. I'm brand new to sql querying, so I'm fairly lost on anything beyond JOINS and WHERE clauses so far. I appreciate the help.
SELECT serviceclass.description, DATEPART(Year, meterread.readdate) AS Year, DATEPART(Month, meterread.readdate) AS Month, SUM(meterread.consumption)
AS Consumption, COUNT(DISTINCT accountservice.account_id) AS Locations
FROM accountservice INNER JOIN
serviceclass ON accountservice.serviceclass_id = serviceclass.serviceclass_id INNER JOIN
meterread ON accountservice.accountservice_id = meterread.accountservice_id
WHERE (#Year = DATEPART(Year, meterread.readdate))
GROUP BY serviceclass.description, DATEPART(Year, meterread.readdate), DATEPART(Month,meterread.readdate)
ORDER BY Year, Month

Answer 1

You can achieve this by using a CASE statement.
replace serviceclass.desription in your SELECT and GROUP BY clauses with
CASE
WHEN serviceclass.description IN ('Desc1','Desc2','Desc3')
THEN 'JointDescription'
ELSE serviceclass.description
END
and also update the Desc 1 ~ 3 with your three description and updated the joint description to what you want it to be.
RESULT:
SELECT CASE
WHEN serviceclass.description IN ('Desc1','Desc2','Desc3')
THEN 'JointDescription'
ELSE serviceclass.description
END as description,
DATEPART(Year, meterread.readdate) AS Year,
DATEPART(Month, meterread.readdate) AS Month,
SUM(meterread.consumption) AS Consumption,
COUNT(DISTINCT accountservice.account_id) AS Locations
FROM accountservice
INNER JOIN serviceclass ON accountservice.serviceclass_id = serviceclass.serviceclass_id
INNER JOIN meterread ON accountservice.accountservice_id = meterread.accountservice_id
WHERE (#Year = DATEPART(Year, meterread.readdate))
GROUP BY CASE
WHEN serviceclass.description IN ('Desc1','Desc2','Desc3')
THEN 'JointDescription'
ELSE serviceclass.description
END,
DATEPART(Year, meterread.readdate),
DATEPART(Month,meterread.readdate)
ORDER BY Year, Month

Answer 2

You would typically do this with a case statement that would go in both the select and group by statements. Something like:
select (case when serviceclass.description = 'A' then 'B'
else serviceclass.description
end) as description
. . . -- fill in with query here
group by (case when serviceclass.description = 'A' then 'B'
else serviceclass.description
end), . . . -- rest of query goes here