Question

Is this SQL query statement:

SELECT p.id, p.[name], SUM(ps.sales_amount) AS GROSS_SALES
FROM products p
  LEFT OUTER JOIN product_sales ps ON p.id = ps.product_id
GROUP BY p.id, p.[name]

better than:

SELECT SUM([t2].[value]) AS [SalesAmount], [t2].[id] AS [ProductId], [t2].[name] AS [ProductName]
FROM (
    SELECT (
        SELECT SUM([t1].[sales_amount])
        FROM [dbo].[product_sales] AS [t1]
        WHERE [t1].[product_id] = [t0].[id]
        ) AS [value], [t0].[id], [t0].[name]
    FROM [dbo].[products] AS [t0]
    ) AS [t2]
GROUP BY [t2].[id], [t2].[name]

The second one is a result of a LINQ2SQL query. Still finding a way to re-write the LINQ expression…

What would be the most optimized SQL query in the example?

Your thoughts? Thanks!

Answer 1

The real answer is that you can’t tell without measuring. Even then you don’t really know unless you measure on a realistic dataset.

Check out this blog post (not by me) where the generated sql looked worse but performed much better. His example specifically deals with subselects vs joins so I think it is very relevant to your situation.