Monday, May 03, 2010

IF EXISTS in SQL

For some reason, I always forget the syntax to using IF EXISTS in SQL. Here is a posting that spells it out nicely (http://blogs.msdn.com/miah/archive/2008/02/17/sql-if-exists-update-else-insert.aspx)


SQL: If Exists Update Else Insert

This is a pretty common situation that comes up when performing database operations.  A stored procedure is called and the data needs to be updated if it already exists and inserted if it does not.  If we refer to the Books Online documentation, it gives examples that are similar to:
IF EXISTS (SELECT * FROM Table1 WHERE Column1='SomeValue')
    UPDATE Table1 SET (...) WHERE Column1='SomeValue'
ELSE
    INSERT INTO Table1 VALUES (...)
This approach does work, however it might not always be the best approach.  This will do a table/index scan for both the SELECT statement and the UPDATE statement.  In most standard approaches, the following statement will likely provide better performance.  It will only perform one table/index scan instead of the two that are performed in the previous approach.

UPDATE Table1 SET (...) WHERE Column1='SomeValue'
IF @@ROWCOUNT=0
    INSERT INTO Table1 VALUES (...)
The saved table/index scan can increase performance quite a bit as the number of rows in the targeted table grows.

No comments: