Why does sql-server prevent inserting in WHEN MATCHED of merge?

Question

Anyone know why sql server prevents inserting from within the WHEN MATCHED clause of a MERGE statement? I understand that the documentation only allows updates or deletes, I'm wondering why this is the case so I can understand merge better.

Look at this post for an example.

Answer 1

If you are trying to merge your source to your target, it does not make sense to insert a line if it was found in the target. You may want to update or delete it though. Inserting what is already there would create duplicates.

Answer 2

As you want to INSERT when you find a MATCH, i presume the condition of the ON-clause is met but another field is different. Consider including this field into the ON-clause with AND to differentiate between present rows and to be inserted rows.