SQL: how to compare corrosponding values two rows from same column?

Question

I have two tables:

VendorTrading;
id  date
1   01-01-2015
2   01-01-2015

CustomerProducts;
id tradeID ProductName Quantity
1  1       XYZ         20
2  2       ABC         30

I need to make comparison of two product's quantity, of names will be given as parameters, on a particular date. here is my query so far,

Select sum(Cp.ProductQuantity) 
FROM VendorTrading VT inner join CustomerProducts CP on VT.Id = CP.VendorTradingId
WHERE  CP.ProductName = ISNULL ('XYZ', CP.ProductName) and VT.Tradedate = isnull('2015-01-20',VT.Tradedate)

which only returns sum of anyone's quantity. How can I achieve that result? I need to put them on a graph in crystal report. Further more I need to vice versa of same thing but with Two dates but one particular product.

Answer 1

If you want to return sum of anyone's quantity, you should use group by.

Select CP.ProductName,sum(Cp.ProductQuantity) 
FROM VendorTrading VT inner join CustomerProducts CP 
ON VT.Id = CP.VendorTradingId
WHERE  CP.ProductName = ISNULL('XYZ', CP.ProductName)
AND VT.Tradedate = isnull('2015-01-20',VT.Tradedate)
AND CP.ProductName in ('xyz','abc')
GROUP BY CP.ProductName

Answer 2

If you want compare more than 2 products, the query must be use group by. Answer of @Loser is good for your question so far. Maybe you should give us more detail of your expected result.