Question

SELECT C.* FROM StockToCategory STC 
INNER JOIN Category C ON STC.CategoryID = C.CategoryID 
WHERE STC.StockID = @StockID

VS

SELECT * FROM Category
WHERE CategoryID IN
    (SELECT CategoryID FROM StockToCategory WHERE StockID = @StockID)

Which is considered the correct (syntactically) and most performant approach and why?

The syntax in the latter example seems more logical to me but my assumption is the JOIN will be faster.

I have looked at the query plans and havent been able to decipher anything from them.

Query Plan 1
Query Plan 2

Answer 1

The two syntaxes serve different purposes. Using the Join syntax presumes you want something from both the StockToCategory and Category table. If there are multiple entries in the StockToCategory table for each category, the Category table values will be repeated.

Using the IN function presumes that you want only items from the Category whose ID meets some criteria. If a given CategoryId (assuming it is the PK of the Category table) exists multiple times in the StockToCategory table, it will only be returned once.

In your exact example, they will produce the same output however IMO, the later syntax makes your intent (only wanting categories), clearer.

Btw, yet a third syntax which is similar to using the IN function:

Select ...
From Category
Where Exists    (
                Select 1
                From StockToCategory
                Where StockToCategory.CategoryId = Category.CategoryId
                    And StockToCategory.Stock = @StockId
                )