case within case using mysql - mysql

Question

I have 3 columns doc_date, chem_date, st_date
all 3 columns belong to different tables:
doc
doc_date count(x)
01-02-2012 2
02-02-2012 3
chem
chem_date count(x)
04-02-2012 1
06-02-2012 0
stock
st_date count(x)
01-02-2012 1
03-02-2012 5
I want to write select clause like this
case doc_date
when '01' then count(x),
case chem_date
when '01' then count(x),
case st_date
when '01' then count(x)
end as '01',
case doc_date
when '02' then count(x),
case chem_date
when '02' then count(x),
case st_date
when '02' then count(x)
end as '02',
....up to 31 days
If some case statements have an entry on same date e.g if doc_date and st_date both exist on '01-02-2012' then their respective count should be added to the final count(x) means count(x) + count(x)
So, the answer should be:
01 02 03 04 05 06 07 08 09 10 11 12.. up to 31 days
3 3 5 1 0 count(x) value for particular date
Explanation of the output: Here, the starting value for date 01 is 2 for doc table and for stock table the value is 1. So, the final value will become addition of them which is 3
The same rule will be applicable for others.
Any suggestions how can I achieve this output in SQL? Or any other way?

Answer 1

Here's a pseudo code showing how it can be done:
select
case when date_format(doc_date,"%d") = '01' then sum_x else null end as '01',
case when date_format(doc_date,"%d") = '02' then sum_x else null end as '02',
--and etc
from (
select doc_date, sum(doc_x) as sum_x --sum the x value
from
(
select doc_date,doc_x --combine all 3 table stuff
from doc a
union all
select chem_date,chem_x
from chem a
union all
select st_date,st_x
from st a
) h
group by doc_date --use first col name as the union's col name
) g
limit 1,1
If the doc, chem and st are combined using joins and have different schema, you may need to do pivot, either using multiple join to itself or use case... when...:
select
case when date_format(doc_date,"%d") = '01' then sum_x else null end as '01',
case when date_format(doc_date,"%d") = '02' then sum_x else null end as '02',
--and etc
from (
select doc_date,
a_x + b_x + c_x as sum_x --sum them according to date
from (select ... from doc where ...) a
left join (select ... from chem where ...) b on a.doc_date=b.doc_date and --some join condition
left join (select ... from st where ...) c on a.doc_date=c.doc_date and --some join condition
) g
limit 1,1
I am working with an ambiguous question here, so clarify as needed,