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Chapter 5: Problem 4

What is the difference between a key and a superkey?

Short Answer

Expert verified
The difference between a key and a superkey in a database is that a key is the minimal set of attribute(s) that can uniquely identify a tuple, while a superkey, although also being able to uniquely identify a tuple, can include extra attributes and therefore is not necessarily minimal.

Step by step solution

01

Understand the concept of a key in a database

A key in a database is a set of one or more attributes (columns) that can uniquely identify a tuple (row) in a relation (table). For example, in a student table, a student ID could be a key as it is unique for every student.
02

Understand the concept of a superkey

A superkey in a database is a set of one or more keys which can uniquely identify a tuple (row) in a relation (table). However, a superkey differs from a key in that it can contain additional attributes that are not necessary for uniqueness. For instance, continuing with our student example, a superkey could be a combination of student ID, name, and class as this set will also uniquely identify a student.
03

Summarize the difference between a key and a superkey

In essence, the difference between a key and a superkey is that a key is the minimal set of attribute(s) that can uniquely identify a tuple, meaning it doesn’t contain any extraneous attributes. On the other hand, a superkey contains one or more keys and can have additional attributes, and so it is not necessarily minimal.

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Most popular questions from this chapter

, Cname, City) ORDER(Order#, Odate, Custw, Ord Amt) ORDER_I… # Consider the following six relations for an order-processing database application in a company: CUSTOMER(Cust#, Cname, City) ORDER(Order#, Odate, Custw, Ord Amt) ORDER_ITEM(Order#, Item#, C2ty) ITEM(Item#, Unicprice) SHIPMENT(Order#, Warehouse#, Ship_date) WAREHousE(Warehouse#, City) Here, Ord_Amt refers to total dollar amount of an order; Odate is the date the order was placed; Ship_date is the date an order is shipped from the warehouse. Assume that an order can be shipped from several warehouses. Specify the foreign keys for this schema, stating any assumptions you make.

Why are duplicate tuples not allowed in a relation?

Discuss the entity integrity and referential integrity constraints. Why is each considered important?

, Univ Section«, InstructorName, Semester, BuildingCode, Roome, TimePeriod, Weekdays, CreditHours). This represents classes taught in … # Consider the relation CLASs(Course#, Univ Section«, InstructorName, Semester, BuildingCode, Roome, TimePeriod, Weekdays, CreditHours). This represents classes taught in a university, with unique Univ_Section#. Identify what you think should be various candidate keys, and write in your own words the constraints under which each candidate key would be valid.

Consider the following relations for a database that keeps track of auto sales in a car dealership (Option refers to some optional equipment installed on an auto): cAR(Serial-No, Model, Manufacturer, Price) OPTIoNs(Serial-No, Option-Name, Price) sALEs(Salesperson-id, Serial-No, Date, Sale-price) sALEsPERsoN(Salesperson-id, Name, Phone) First, specify the foreign keys for this schema, stating any assumptions you make. Next, populate the relations with a few example tuples, and then give an example of an insertion in the SALES and SALESPERSON relations that violates the referential integrity constraints and of another insertion that does not.
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