将基于另一个表值修改的表与 MySQL 中的表连接起来

Question

I need some help with the MySQL query for the following problem.

I have two tables A (id, ref_id) and B (ref_id (primary key), class, pulse) where A.ref_id is present in B.ref_id. ref_id has integer values starting from 1 and is the P_k of B.

A.id | A.ref_id
  1  |  2
  2  |  3

B.ref_id | B.pulse | B.class
     1   |  100    | X
     2   |  50     | Y
     3   |  100    | X

What I want to do is create table which has the sum of all pulse values corresponding to all B.ref_id less than the A.ref_id. Also, only those pulse should be considered whose B.class is the same as the B.class of the B.ref_id referred from A.ref_id. For example,

A.ref_id | sum
  2      | 50
  3      | 200

In this example, A.ref_id having value 2 refers to the corresponding B.ref_id and thus has B.class Y. Since I want sum of pulse values for all B.ref_id<=2 and there are no other rows with B.class=Y, the value of sum is 50. However, for A.ref_id=3, the corresponding B.class is X and I want the sum of all B.pulse that have occured till now with the same class. Thus the value of sum would be 100 (from B.ref_id=1) + 100 (from B.ref_id=A.ref_id=3) = 200.

Can someone please help me out with this?

I need something in MySQL like:

SELECT A.ref_id, sum 
FROM A
JOIN (SELECT class, SUM(B.pulse) as sum 
      FROM B WHERE B.ref_id<=A.ref_id 
      GROUP BY B.class) t
ON t.class= (SELECT class FROM B WHERE B.ref_id=A.ref_id)

The above doesn't work because I can't refer A.ref_id inside a sub-query (t). And joining without that condition doesn't help me with what I want to do.

Thank you!!

Answer 1

You can do it by joining 2 copies of B to A:

SELECT a.ref_id, 
       SUM(b2.pulse) `sum`
FROM TableA a 
INNER JOIN TableB b1 ON b1.ref_id = a.ref_id
INNER JOIN TableB b2 ON b2.ref_id <= b1.ref_id AND b2.class = b1.class
GROUP BY a.ref_id

See the demo .
Results:

ref_id	sum
2	50
3	200

Answer 2

You can use a correlated subquery:

select a.*,
       (select sum(b2.pulse)
        from b2
        where b2.class = b.class and
              b2.ref_id <= a.ref_id
       )
from a join
     b
     on a.ref_id = b.ref_id;

Or, use window functions with the join :

select a.*, b.pulse_sum           
from a join
     (select b.*, 
             sum(pulse) over (partition by class order by ref_id) as pulse_sum
      from b
     ) b
     on a.ref_id = b.ref_id;