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

Assuming that you want to insert/update single row, most optimal approach is to use SQL Server's REPEATABLE READ transaction isolation level:

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
BEGIN TRANSACTION

    IF (EXISTS (SELECT * FROM myTable WHERE key=@key)
        UPDATE myTable SET ...
        WHERE key=@key
    ELSE
        INSERT INTO myTable (key, ...)
        VALUES (@key, ...)

COMMIT TRANSACTION

This isolation level will prevent/block subsequent repeatable read transactions from accessing same row (WHERE key=@key) while currently running transaction is open. On the other hand, operations on another row won't be blocked (WHERE key=@key2).