Question

I have a table, called Prices composed of a few fields, two of which is ProductId and PriceType. Some products have both Wholesale price (PriceType = W) and Retail price (PriceType = R), i.e. they have two records with the same product id.

I would like to display to the user those products with no retail price (i.e. those with only one record W).

How should I write my query to do this?

I have used something like

Select * from Products 
INNER JOIN Prices ON Products.Id = Prices.ProductId 
WHERE ProductId NOT IN 
  (SELECT ProductId FROM Prices WHERE PriceType="R") 

but this is executing very slow. I have checked and verified that the part taking a long time is really the NOT IN clause.

Any ideas?

Answer 1

SELECT * 
FROM Products 
INNER JOIN Prices 
        ON Products.Id = Prices.ProductId 
       AND Prices.PriceType = 'W'

This will filter out the Prices at the join so you never have any PriceType='R' records joined.

To filter out those records that do not have an ‘R’ at all, you can use the following query, which removes the sub-select on your where clause

SELECT 
    Products.ProductId
FROM 
    Products 
    INNER JOIN Prices A
        ON Products.ProductId = A.ProductId 
        AND A.PriceType = 'W'
    LEFT JOIN Prices B
        ON A.ProductId = B.ProductId 
        AND B.PriceType <> A.PriceType    
WHERE
    A.PriceType = 'W'
    AND B.PriceId is null

To ensure this runs as fast as possible, ensure you add an index to your Prices table that includes both the ProductId and PriceType columns