Exists or not for the current customer

Sometimes we only need to know whether a match exists, not join every order. Correlate the test to the current customer, keep one row, and distinguish “no exception” from “activity and no exception”.

EXISTS is TRUE when a subquery returns at least one row; NOT EXISTS when none. Correlation to the outer customer defines whose matches are tested.

Exists or not for the current customer: EXISTS is TRUE when a subquery returns at least one row; NOT EXISTS when none. Correlation to the outer customer defines whose matches are tested.