Question

I wasn’t too sure how to present this question to keep it consise while still giving plenty of info to work with. If any one needs more info/tables let me know and I’ll be happy to edit.

I’m trying to query a database and need to join several tables to get the data I want.

The query is: Show me amount of items from supplier x that were sold between two dates and how many of them are currently in stock.

The SQL for this is as follows:

SELECT p.id as product_id, p.description as description, SUM(col.quantity) as qty,   SUM(s.stocklevel) as stocklevel, sup.name as supplier from products as p
LEFT JOIN customerorderlines as col on col.product_id = p.id
LEFT JOIN customerorders as co on co.id = col.customerorder_id
LEFT JOIN stock as s on s.product_id = p.id
LEFT JOIN suppliers as sup on sup.id = p.supplier_id
WHERE co.orderdate BETWEEN '2009-07-01' AND '2009-08-01'
AND p.supplier_id = 51
GROUP by col.product_id
ORDER by SUM(col.quantity)
DESC

Here is one of the rows it retuns:

product_id description                 qty   stocklevel  supplier
24376      Streaker Wax Paper Strips   330   3510        Rand Rocket Ltd

Notice stocklevel returning 3510.

Now lets take a look at this on its own:

SELECT SUM(stocklevel) from stock where product_id = 24376

SUM(stocklevel)
90

With my limited mysql knowledge, I have no idea why it is doing this. Any suggestions are extremly appreciated.

Thanks in advance.

Answer 1

You are most likely counting several product_id’s stocklevels multiple times.

One solution would be to pre-compute the stocklevel for each product_id and join this with your original query.

SELECT    p.id as product_id
          , p.description as description
          , SUM(col.quantity) as qty
          , sl.stocklevel as stocklevel
          , sup.name as supplier 
from      products as p 
          INNER JOIN (
            SELECT    product_id, SUM(stocklevel) as stocklevel
            from      stock 
            GROUP BY  product_id
          ) sl ON sl.product_id = p.product_id
          LEFT JOIN customerorderlines as col on col.product_id = p.id 
          LEFT JOIN customerorders as co on co.id = col.customerorder_id 
          LEFT JOIN stock as s on s.product_id = p.id 
          LEFT JOIN suppliers as sup on sup.id = p.supplier_id 
WHERE     co.orderdate BETWEEN '2009-07-01' AND '2009-08-01' 
          AND p.supplier_id = 51 
GROUP by  col.product_id, sl.stocklevel
ORDER by  SUM(col.quantity) DESC