Assuming that all the tables have been created as given in the relational schema, write SQL code to make the following changes in the structure of the tables:
(i) Make the employee_id the Primary Key of the EMPLOYEES table
(ii) Make the department_id and the manager_id the foreign key of the Employee table assuming that the department_id has been declared as the primary key of the DEPARTMENTS.
(iii) Add a check constraint in the Job_History table so that the end_date will always be greater than the start_date.
(iv) Add a unique constraint in the Employee table so that each employee will have a unique email address.