8. MGR is the employee’s manager, who must himself be an employee. The
column is defined in the same way as the table’s primary key column
of EMPNO. The constraint EMP_MGR_FK defines this column as a self-
referencing foreign key, so any value entered must refer to an already-extant
row in EMP (though it is not constrained to be not null, so it can be left blank).
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