ALTER TABLE Employee
ADD FOREIGN KEY (DeptNo) REFERENCES Department(DeptNo);
ALTER TABLE 'table_name'
ADD CONSTRAINT 'constraint_name' FOREIGN KEY 'foreign_key' REFERENCES 'column_name'('primary_key');
ALTER TABLE message ADD FOREIGN KEY (sender) REFERENCES users;
ALTER TABLE message ADD FOREIGN KEY (sender) REFERENCES users;
ALTER TABLE message ADD FOREIGN KEY (sender) REFERENCES users;