SQL Efficiency: WHERE IN Subquery vs. JOIN then GROUP

Question

As an example, I want to get the list of all items with certain tags applied to them. I could do either of the following:

SELECT Item.ID, Item.Name
FROM Item
WH         


        

Answer 1

run this:

SET SHOWPLAN_ALL ON

then run each version of the query

you can see if they return the same plan, and if not look at the TotalSubtreeCost on the first row of each and see how different they are.