MYSQL Subtracting two SELECT Queries - mysql

Question

Using MYSQL, I have written two big SELECT queries combined by a UNION, to get 2 rows, where the first row is the count for the current month, and the second row is the count for the previous month. The Query is as follows:
select * from
(select count(*) as type1 from table_x where nationality_id = 23 and month(START_DATE) = month(now())) as t1,
(select count(*) as type2 from table_x where nationality_id = 24 and month(START_DATE) = month(now())) as t2,
(select count(*) as type3 from table_x where nationality_id = 25 and month(START_DATE) = month(now())) as t3,
(select count(*) as type4 from table_x where nationality_id = 26 and month(START_DATE) = month(now())) as t4
UNION
select * from
(select count(*) as type1 from table_x where nationality_id = 23 and month(START_DATE) = month(now() - INTERVAL 1 MONTH)) as t1,
(select count(*) as type2 from table_x where nationality_id = 24 and month(START_DATE) = month(now() - INTERVAL 1 MONTH)) as t2,
(select count(*) as type3 from table_x where nationality_id = 25 and month(START_DATE) = month(now() - INTERVAL 1 MONTH)) as t3,
(select count(*) as type4 from table_x where nationality_id = 26 and month(START_DATE) = month(now() - INTERVAL 1 MONTH)) as t4
I want to add a third row, which is the difference between row 2 and row 1.
How can I do this with my current query?

Answer 1

You are obviously doing a compare between current and prior month. So, I would start with my inner pre-query aggregate getting only those transactions >= the first of the prior month AND the records within the nationality IDs you are looking for.
The inner date_sub() of DAYOFMONTH() -1 day gives you the first of the CURRENT month. By subtracting one more month, gives you the first of the LAST month.
Now you can aggregate the totals per nationality compared to current month or not. That inner query gives you all begin and end counts. Now that is wrapped to the outer and you can get all the counts in addition to the differences... Obviously you can change the column names respectively.
select
PQ.*,
PQ.ThisMonth23 - PQ.LastMonth23 = Diff23,
PQ.ThisMonth24 - PQ.LastMonth24 = Diff24,
PQ.ThisMonth25 - PQ.LastMonth25 = Diff25,
PQ.ThisMonth26 - PQ.LastMonth26 = Diff26
from
( select
sum( case when t.Nationality_id = 23 and month( t.StartDate ) = month( now()) then 1 else 0 end ) ThisMonth23,
sum( case when t.Nationality_id = 24 and month( t.StartDate ) = month( now()) then 1 else 0 end ) ThisMonth24,
sum( case when t.Nationality_id = 25 and month( t.StartDate ) = month( now()) then 1 else 0 end ) ThisMonth25,
sum( case when t.Nationality_id = 26 and month( t.StartDate ) = month( now()) then 1 else 0 end ) ThisMonth26,
sum( case when t.Nationality_id = 23 and month( t.StartDate ) != month( now()) then 1 else 0 end ) LastMonth23,
sum( case when t.Nationality_id = 24 and month( t.StartDate ) != month( now()) then 1 else 0 end ) LastMonth24,
sum( case when t.Nationality_id = 25 and month( t.StartDate ) != month( now()) then 1 else 0 end ) LastMonth25,
sum( case when t.Nationality_id = 26 and month( t.StartDate ) != month( now()) then 1 else 0 end ) LastMonth26
from
table_x t
where
t.StartDate >= date_sub( date_sub( t.StartDate, interval DAYOFMONTH( t.StartDate ) -1 DAY ), interval 1 MONTH )
AND t.Nationality_id IN ( 23, 24, 25, 26 )
) PQ
I would just add that your query might be getting more than you think... You are asking for ALL records Ex: January REGARDLESS of the year, compared to ALL records December REGARDLESS of the year because all you are qualifying is based on the MONTH() and no YEAR() consideration. I am explicitly querying back only current and prior month.