Query Returning SUM of Quantity or 0 if no Quantity for a Product at a given date - sql

Question

I have the following 4 tables:
----------
tblDates:
DateID
30/04/2012
01/05/2012
02/05/2012
03/05/2012
----------
tblGroups:
GroupID
Home
Table
----------
tblProducts:
ProductID GroupID
Chair Home
Fork Table
Knife Table
Sofa Home
----------
tblInventory:
DateID ProductID Quantity
01/05/2012 Chair 2
01/05/2012 Sofa 1
01/05/2012 Fork 10
01/05/2012 Knife 10
02/05/2012 Sofa 1
02/05/2012 Chair 3
03/05/2012 Sofa 2
03/05/2012 Chair 3
I am trying to write a query that returns all Dates in tblDates, all GroupIDs in tblGroups and the total Quantity of items in each GroupID.
I manage to do this but only get Sum(Quantity) for GroupID and DateID that are not null. I would like to get 0 instead. For exemple for the data above, I would like to get a line "02/05/2012 Table 0" as there is no data for any product in "Table" group on 01/05/12.
The SQL query I have so far is:
SELECT tblDates.DateID,
tblProducts.GroupID,
Sum(tblInventory.Quantity) AS SumOfQuantity
FROM (tblGroup
INNER JOIN tblProducts ON tblGroup.GroupID = tblProducts.GroupID)
INNER JOIN (tblDates INNER JOIN tblInventory ON tblDates.DateID = tblInventory.DateID) ON tblProducts.ProductID = tblInventory.ProductID
GROUP BY tblDates.DateID, tblProducts.GroupID;
I reckon I should basically have the same Query on a table different from tblInventory that would list all Products instead of listed products with 0 instead of no line but I am hesitant to do so as given the number of Dates and Products in my database, the query might be too slow.
There is probably a better way to achieve this.
Thanks

Answer 1

To get all possible Groups and Dates combinations, you'll have to CROSS JOIN those tables. Then you can LEFT JOIN to the other 2 tables:
SELECT
g.GroupID
, d.DateID
, COALESCE(SUM(i.Quantity), 0) AS Quantity
FROM
tblDates AS d
CROSS JOIN
tblGroups AS g
LEFT JOIN
tblProducts AS p
JOIN
tblInventory AS i
ON i.ProductID = p.ProductID
ON p.GroupID = g.GroupID
AND i.DateID = d.DateID
GROUP BY
g.GroupID
, d.DateID

Answer 2

use the isnull() function. change your query to:
SELECT tblDates.DateID, tblProducts.GroupID,
**ISNULL( Sum(tblInventory.Quantity),0)** AS SumOfQuantity
FROM (tblGroup INNER JOIN tblProducts ON tblGroup.GroupID = tblProducts.GroupID)
INNER JOIN (tblDates INNER JOIN tblInventory ON tblDates.DateID = tblInventory.DateID)
ON tblProducts.ProductID = tblInventory.ProductID
GROUP BY tblDates.DateID, tblProducts.GroupID;

Answer 3

You don't say which database you're using but what I'm familiar with is Oracle.
Obviously, if you inner join all tbales, you will not get the rows containing nulls. If you use an outer join to the inventory table you also have a problem because the groupid column is not listed in tblinventory and you can only outer join to one table.
I'd say you have two choices. Use a function or have a duplicate of the groupid column in the inventory table.
So, a nicer but slower solution:
create or replace
function totalquantity( d date, g varchar2 ) return number as
result number;
begin
select nvl(sum(quantity), 0 )
into result
from tblinventory i, tblproducts p
where i.productid = p.productid
and i.dateid = d
and p.groupid = g;
return result;
end;
select dateid, groupid, totalquantity( dateid, groupid )
from tbldates, tblgroups
Or, if you include the groupid column in the inventory table. (You can still quarantee integrity using constraints)
select d.dateid, i.groupid, nvl(i.quantity, 0)
from tbldates d, tblinventory i
where i.dateid(+) = d.dateid
group by d.dateid, g.groupid
Hope this helps,
Gísli