Question

Consider the following schema for customer: (id, name, street, city, state, zip, phone), in which customers are uniquely determined by their name and phone or by their id. How many super keys are there for this relation? Justify your answer.

Answer 1

Answer:

Super key is a set of attributes that uniquely identifies all attributes in a relation.Super keys are more than candidate key.

Explanation:

For example in the given schema where

Customer: (id , name , street , city,state,zip,phone)

We have a relation we check super keys by following dependencies

Functional Dependencies                          Super Key

id                                                                     YES

name                                                               YES

id , name -> street , city,state,zip,phone       YES

id , street  ->name , city,state,zip,phone       YES

id,city ->name , street ,state,zip,phone         YES

id,state ->name , street ,city,zip,phone         YES

id,phone  ->name , street ,state,zip,city        YES

id , name , street                                             YES

id , name , street , city                                    YES

id , name , street , city,state                          YES

id , name , street , city,state,zip,phone         YES

All these are uniquely identified

All these are super key but in reverse they are not super key such as

street , city,state,zip,phone->id , name        No