[Zope] testing for existence of value in SQLtable

Is there an easy way to test whether a value exists in a SQL table, so that I can do something like this in a ZSQL Method: <dtml-if> [here follows the test for the existence of a value in a table] insert items into table </dtml-else> update item in table </dtml-if> This problem probably came along before on the lists, but I can't find it anymore... Rik