Illustrate the difference between relation schema and relation instance.

Database Management System >   DBMS Relational Databases >   Relational Database Design  

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Answer:

A relation schema (also termed as relation intension) depicts the attributes of the table. It consists of a relation name R and a set of attributes (or fields) A1, A2,……, An. It is represented by R (A1, A2,……, An) and is used to describe a relation R. To understand the concept of relation schema, consider the book, publisher and author information stored in an Online Book database with the schema of the relations given as:

BOOK (ISBN, Book title, Category, Price, Copyright date, Year,

Page count, P ID)

AUTHOR (A_ID, Aname, City, State, Zip, Phone, URL)

PUBLISHER (P_ID, Pname, Address, State, Phone, Email_id)

In these statements, ISBN, Book_title, Category, Price, Copyright_date, Year, Page_count and P_ID are the attributes of relation BOOK. A_ID, Aname, City, State, Zip, Phone and URL are the attributes of relation AUTHOR. P_ID, Pname, Address, State, Phone and Email_id are the attributes of relation PUBLISHER.

A relation instance (also termed as relation extension) is a two-dimensional table with a time-varying set of tuples. A relation instance (or relation) r of the relation schema R (A₁, A₂,……, A_n) is a set of n-tuples t. It is also denoted by r(R). A relation instance is an ordered set of attributes values v that belongs to the domain D and it can be denoted as r = {t1, t2,……, tm} where, t = {v1,……, vn). It can be considered as a table, which is a collection of tuples having same number of attributes.

Above figure shows an instance B1 of BOOK relation which contains three tuples and eight attributes. If the tuple t denotes the first tuple of the relation, the notation t[ISBN] refers to the value of t on the ISBN attribute. Hence, t[ISBN] = "001-354-921-1", t[Book_title] = "Ransack" and so on.

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