sqlite select: grouping in one line, 12 entries - sql

Question

I have a table with fields:
Client_ID, Date, Value
where there is an entry for each of the 12 months of a year (i.e., 12 entries for each client).
I would like to create a table with just one row per Client_ID that contains all the values from each months. Something like:
Client_ID, Date_January, Value_January, Date_February, Value_February, ........, Date_December, Value_December
Can anyone help me with the query?
This is what I'm trying to do (not working...):
select
Client_Id,
case when ((strftime('%m', Date) = '01')) then
Date as Date_January,
Value as Value_January,
else null end
case when ((strftime('%m', Date) = '01')) then
Date as Date_February,
Value as Value_February,
else null end
....
from Test_Table
where
strftime('%Y', Date) = '2013'
;

Answer 1

First you need to untangle your case constructs as they generate a single value. Use:
case
when ((strftime('%m', Date) = '01')) then Date
else null
end as Date_January,
case when ((strftime('%m', Date) = '01')) then Value
else null
end as Value_January,
Then, if you want one row per client, use GROUP BY ClientID.
The third issue is how to aggregate all the Date_January columns into one row. If you really know that there is exactly one row per month per client, you can use MAX() knowing that the not null value will be higher than the NULL values:
select
Client_Id,
MAX(case
when ((strftime('%m', Date) = '01')) then Date
else null
end) as Date_January,
MAX(case when ((strftime('%m', Date) = '01')) then Value
else null
end) as Value_January,
MAX(case
when ((strftime('%m', Date) = '02')) then Date
else null
end) as Date_February,
MAX(case when ((strftime('%m', Date) = '02')) then Value
else null
end) as Value_February,
....
from Test_Table
where
strftime('%Y', Date) = '2013'
group by Client_Id;