Solutions for INSERT OR UPDATE on SQL Server

Question

Assume a table structure of MyTable(KEY, datafield1, datafield2...).

Often I want to either update an existing record, or insert a new record if it does

Answer 1

If going the UPDATE if-no-rows-updated then INSERT route, consider doing the INSERT first to prevent a race condition (assuming no intervening DELETE)

INSERT INTO MyTable (Key, FieldA)
   SELECT @Key, @FieldA
   WHERE NOT EXISTS
   (
       SELECT *
       FROM  MyTable
       WHERE Key = @Key
   )
IF @@ROWCOUNT = 0
BEGIN
   UPDATE MyTable
   SET FieldA=@FieldA
   WHERE Key=@Key
   IF @@ROWCOUNT = 0
   ... record was deleted, consider looping to re-run the INSERT, or RAISERROR ...
END

Apart from avoiding a race condition, if in most cases the record will already exist then this will cause the INSERT to fail, wasting CPU.

Using MERGE probably preferable for SQL2008 onwards.