Database Redesign
The NOT EXISTS keyword will be true if:

both of these two conditions are met.
neither of these two conditions is met.
any row in the subquery meets the condition.
all rows in the subquery fail the condition.

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Database Redesign
Before any changes to database structure are attempted one should first:

test any changes on a test database only.
create a complete backup of the operational database only.
All of these
clearly understand the current structure and contents of the database only.

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