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Check whether privilege has the "not null" restriction. If so it always has to have a value and can never be null.
A different reason could be a wrong implementation of a foreign key. Check whether you always correctly insert/update in both tables.
Nothing is wrong with the database. When it gives you those errors, it's telling you that you are trying to input "nothing" (null) into a column that doesn't allow it. Fix it by removing the not null constraint or by passing a value to that field.
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