Database Redesign
To drop a column that is used as a foreign key, first:

All of these
drop the table containing the foreign key.
drop the foreign key constraint.
drop the primary key.

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Database Redesign
The EXISTS keyword will be true if:

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

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