Sql Server 2005 - Insert if not exists

Question

There is lots of information in the internet regarding this common \"problem\".

Solutions like:

IF NOT EXISTS() BEGIN INSERT INTO (...) END
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Answer 1

To answer the updated question repeatable read would still not be sufficient.

It is holdlock / serializable level that you would need.

You are trying to prevent phantoms (where on the first read no rows met the criteria so the NOT EXISTS returns true but subsequently a concurrent transaction inserts a row meeting it)

Answer 2

With TRY/CATCH you can avoid the extra read

BEGIN TRY
   INSERT etc
END TRY
BEGIN CATCH
    IF ERROR_NUMBER() <> 2627
      RAISERROR etc
END CATCH

• A NOT EXISTS will read the table, whether in the IF or WHERE
• The INSERT requires a read to check uniqueness

If you can discard duplicates, this is a highly scalable technique

Links:

• See my answers here: Only inserting a row if it's not already there and SQL Server 2008: INSERT if not exits, maintain unique column
• If you requires UPDATEs too: Which is the best choice in delete-insert vs if-update else-insert?