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Based on the structure of the order items table, which of the following commands will add a NOT NULL constraint to the ISBN column?
a) ALTER TABLE order_items MODIFY COLUMN isbn NOT NULL;
b) ALTER TABLE order_items ADD CONSTRAINT isbn_not_null CHECK (isbn IS NOT NULL);
c) ALTER TABLE order_items ALTER COLUMN isbn SET NOT NULL;
d) ALTER TABLE order_items CHANGE COLUMN isbn isbn VARCHAR(50) NOT NULL;



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