merge - skip update if records > X

H All,

I'm doing a merge update/insert but want to verify that a condition doesn't exist before doing so i.e. the number of rows does not exceed a number e.g. 1000

so -
If the number of entries that will be updated/inserted from table a to table > 1000 send a specific message to the output

Some suggestions on best way to do this would be appreciated.

Thanks,
Mark
(newbie)