when i joined the results in the parent table, it affects one item which it should not be affected

Question

I have these 3 tables here:

 producttbl:
     p_id        p_name          price
    ANK001       ANKLET           200
    BRC001       BRACELET         100


    stocks_add:
     p_id          qty
    ANK001         200
    ANK001          50


    stocks_rel:
     p_id           qty
    ANK001           20

And i have this code right here (which was suggested by @Bohemian though i added some):

 Select p.p_id, p.p_name, p.price,
`coalesce((SELECT sum(a.qty) 
    FROM stocks_add a GROUP BY a.p_id),0)
-coalesce((SELECT sum(r.qty)
    FROM stocks_rel r GROUP BY r.p_id),0)
 as stock_on_hand
from productstbl p
left join stocks_add a
on a.p_id=p.p_id
left join stocks_rel r
on r.p_id=p.p_id
GROUP BY p.p_id`

and when I execute it, it results to this:

p_id    p_name   price   stock_on_hand
ANK001  Anklet    200       230
BRC001  Bracelet  75        230

I'm trying to get the stocks on hand from the second and third table. and link it with products table.

Suppose to be, 230 should be only with ANK001. The desired output should be this one:

   `p_id    p_name   price   stock_on_hand
    ANK001  Anklet    200       230      
    BRC001  Bracelet  75         0  

 `

but the result on the first output tells the other way around. :( What's wrong with the code? please do help me.

Answer 1

I'm not sure what your issue with your query is, but here's how it was easiest for me to write:

select p.*, IFNULL(aqty,0) -  IFNULL(rqty,0) AS stock_on_hand
from productstbl p
left join (
   select p_id, SUM(IFNULL(a.qty,0)) as aqty 
  from stocks_add a
    group by p_id
  ) AS asum 
  on asum.p_id = p.p_id
  left join 
  (
    select p_id, SUM(IFNULL(r.qty,0)) as rqty
    from stocks_rel r 
    group by p_id
  ) AS rsum 
  on rsum.p_id = p.p_id;

and it actually gives the right results! Which is a nice thing for a query to do :)

P_ID    P_NAME  PRICE   STOCK_ON_HAND
ANK001  ANKLET  200 230
BRC001  BRACELET    100 0

http://sqlfiddle.com/#!2/3c778/3