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Mysql query syntax

TableA
--------------------
productID,Name
1,hat
2,cup
3,pen

TableB
----------------------
orderID, productID, qtyOrdered, custID
1,1,10,1
2,1,10,1
3,3,30,1

TableC
----------------
custID, Name
1,jones

select a.productName,c.Name, sum(b.qtyOrdered) as qSum from TableA a
left join TableB b on a.productID=b.productID
left join TableC c on b.custID=c.custID
group by a.productID

returns
Hat,20,Jones
Pen,30,Jones

I want the following result
Hat,20,Jones
Cup,0,Null
Pen,30,Jones

What I want is to return a quantity of zero where no orders were placed for a product?

How do I do this ?
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joomla
Asked:
joomla
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1 Solution
 
warddhoogheCommented:
Assuming you dont want to left join your customer's table (C) the solution could be to do the sum in a subquery inner joining B and C, then left/right join it with your products table (A)
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joomlaAuthor Commented:
Can you give me an example of how to construct that statement ?
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Chris StanyonCommented:
There's a function in MySQL called COALESCE that you can use to return 0 instead of NULL, but the bigger issue is with the logic in your SQL. You are grouping on ProductID, but showing the customer ID, so if 10 different customer each ordered 10 particular products, you show a SUM of 100 for the first customer - not sure that's what you want (you may need to group on the customer ID as well):

SELECT a.productName, COALESCE(SUM(b.qtyOrdered),0) AS qSum, c.Name FROM TableA a
LEFT JOIN TableB b ON a.productID=b.productID
LEFT JOIN TableC c ON b.custID=c.custID
GROUP BY a.productID, b.custID

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joomlaAuthor Commented:
thanks, I didn't know of 'coalesce' and appreciate you pointing out the need for extra grouping.

with this syntax you provided, I would assume that record in tableA without matching record in tableB would not be shown ?
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Chris StanyonCommented:
No. Because of the LEFT join, all records from TableA will be shown (even if they have no matching record on TableB)- that's the whole point of a LEFT join.

In your original question, that's what you asked for:

What I want is to return a quantity of zero where no orders were placed for a product?
0
 
joomlaAuthor Commented:
Brilliant
thanks for your help
regards
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