sql server - SQL Efficiency: WHERE IN Subquery vs. JOIN then GROUP -

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As an example, I want to get a list of all the items that have some tags applied to them. I can do any of the following: 

  Select item. Id, items. Item items from item where item items in item (id) select item items from item TIGID = 57 or item tag. TIGID = 55)

or 

  select item. Id, item. Skip Item ItemTag.ItemID = Item.ID on itemTag WHERE ItemTag.TagID = 57 or item tag. TIGID = 55 Group item item. Id, item. Name

Or something completely different

In general (assuming that there is a general rule), what is a more efficient approach? 

  select Item.ID, ItemName Item with WHERE Item ID IN (item tags from item tag tag = 57 or item tag. TIGID = 55)

or 

  SELECT Item.ID, Item Item Item Description Item item.ItemID = Item.ID, item tag, item tag, item item, tag.agid = 57, or ItemTag.TagID = 55 group item .id

The second D query will not compile, because it gives reference to Item.Name without any group or group.

If we remove GROUP BY from the query:

select items. Id, item. Items from item to item tag. ITAMID = item. Item tags on id where item tag. TIGID = 57 or item tag TIGID = 55

SQL Server

These are still different questions, unless ItemTag.ItemId < / Code> is able to locate a IN position on a UNIQUE / code> a IN column, and it is IN < / Code> will change the status to JOIN .

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

You can traceform another query in JOIN : 

  select item. Id, items. Add items to item (select DISTI Nct item id item item tag where item tag tagID = 57 or item tag .tigid = 55) tags on tags. ITEM = ITEM.ID

But this is a very less efficient IN or EXISTS .

For a more detailed performance comparison of this article in your blog, see:


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