Oracle Creating Constraints

We have the field GENDER and it will be useful to validate inserted data to normalize and have always M or F, so :

alter table DEPENDENTS add constraint DEPENDENTS_CON check(GENDER IN('M','F')) ENABLE

As you can see this constraint is a "check" kind and the allowed values are M or F.
We had an error because there was inserted bad normalized data:
ERROR at line 1:
ORA-02293: cannot validate (FACTURATOR.DEPENDENTS_CON) - check constraint
violated

After changing the data, the constraint was allowed to be activated.

Other type of constraint the FOREIGN KEY:

SQL> alter table dependents add constraint DEPENDENTS_FK FOREIGN KEY(RELATIVEID) REFERENCES EMPLOYEES(EMPLOYEE_ID)ENABLE;

Table altered.

This is useful to link one table field to another simetric one from other table.