When you create a subtype, you create a type hierarchy. In a type hierarchy, each subtype that you create inherits its properties from a single supertype. If you create a named ROW type customer_t under person_t, customer_t inherits all the fields of person_t. If you create another named ROW type, salesrep_t under customer_t, salesrep_t inherits all the fields of customer_t.
Thus, salesrep_t inherits all the fields that customer_t inherited from person_t as well as all the fields defined specifically for customer_t. For a discussion of type inheritance, refer to the IBM Informix Guide to SQL: Tutorial.
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