Flattening two tables and using a group by - mysql

Question

I have two transaction tables like this:
TableA
X R S
71 1 10
71 2 20
71 3 30
72 1 40
72 2 50
72 3 60
TableB
X P Q
71 1 110
71 2 120
71 3 130
73 1 140
73 2 150
73 3 160
I want to flatten the table and execute a query which gives me a result like this:
X S1 S2 S3 Q1 Q2 Q3
71 10 20 30 110 120 130
72 40 50 60 0 0 0
73 0 0 0 140 150 160
where the subscripts in S and Q denote the value of the third column when the second column equals the subscript. For example, S1 denotes the S column value when second column is equal to 1, or Q2 denotes the value of Q when the second column is 2.
I can't figure out what is the best way to about it. Maybe I can use a case statement or may be a subquery. But honestly I have no concrete idea of going about it.

Answer 1

You would want to do this with a join and two aggregations:
select coalesce(a.x, b.x) as x, a.s1, a.s2, a.s3, b.q1, b.q2, b.q3
from (select x,
max(case when r = 1 then s end) as s1,
max(case when r = 2 then s end) as s2,
max(case when r = 3 then s end) as s3
from tableA a
group by x
) a full outer join
(select x,
max(case when p = 1 then q end) as q1,
max(case when p = 2 then q end) as q2,
max(case when p = 3 then q end) as q3
from tableb b
group by x
) b
on a.x = b.x;
EDIT:
If you want to avoid the nested subqueries, you could try:
select coalesce(a.x, b.x) as x,
max(case when r = 1 then s end) as s1,
max(case when r = 2 then s end) as s2,
max(case when r = 3 then s end) as s3
max(case when p = 1 then q end) as q1,
max(case when p = 2 then q end) as q2,
max(case when p = 3 then q end) as q3
from TableA a full outer join
TableB b
on a.x = b.x and a.r = b.p
group by coalesce(a.x, b.x);
It might even be more efficient.

Answer 2

SELECT
ISNULL(A.x, B.x) AS x, SUM(CASE WHEN R = 1 THEN S ELSE 0 END) AS S1,
SUM(CASE WHEN R = 2 THEN S ELSE 0 END) AS S2,
SUM(CASE WHEN R = 3 THEN S ELSE 0 END) AS S3,
SUM(CASE WHEN P = 1 THEN Q ELSE 0 END) AS Q1,
SUM(CASE WHEN P = 2 THEN Q ELSE 0 END) AS Q2,
SUM(CASE WHEN P = 3 THEN Q ELSE 0 END) AS Q3
FROM tableA A FULL JOIN tableB B ON A.X = B.X
GROUP BY ISNULL(A.x, B.x)
T-SQL version