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

SELECT Item.ID, Item.Name
FROM Item
WHERE Item.ID IN (
    SELECT ItemTag.ItemID
    FROM ItemTag
    WHERE ItemTag.TagID = 57 OR ItemTag.TagID = 55)

or

SELECT Item.ID, Item.Name
FROM Item
LEFT JOIN ItemTag ON ItemTag.ItemID = Item.ID
WHERE ItemTag.TagID = 57 OR ItemTag.TagID = 55
GROUP BY Item.ID

Your second query won't compile, since it references Item.Name without either grouping or aggregating on it.

If we remove GROUP BY from the query:

SELECT  Item.ID, Item.Name
FROM    Item
JOIN    ItemTag
ON      ItemTag.ItemID = Item.ID
WHERE   ItemTag.TagID = 57 OR ItemTag.TagID = 55

these are still different queries, unless ItemTag.ItemId is a UNIQUE key and marked as such.

SQL Server is able to detect an IN condition on a UNIQUE column, and will just transform the IN condition into a JOIN.

If ItemTag.ItemID is not UNIQUE, the first query will use a kind of a SEMI JOIN algorithm, which are quite efficient in SQL Server.

You can trasform the second query into a JOIN:

SELECT  Item.ID, Item.Name
FROM    Item
JOIN    (
        SELECT DISTINCT ItemID
        FROMT  ItemTag
        WHERE  ItemTag.TagID = 57 OR ItemTag.TagID = 55
        ) tags
ON      tags.ItemID = Item.ID

but this one is a trifle less efficient than IN or EXISTS.

See this article in my blog for a more detailed performance comparison:

• IN vs. JOIN vs. EXISTS