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3.2. A Short Glossary

To help keep the various database terms clear, let us collect their definitions in one place.


Entity

An object about which the database is designed to store information. Example: a book; that is, an ISBN, a title, and a price, as in:

0-12-333433-3, On Liberty, $25.00


Attribute

A property that (partially or completely) describes an entity. Example: title.


Entity Class

An abstract group of entities, with a common description. Example: the entity class Books, representing all books in the universe.


Entity Set

The set of entities from a given entity class that are currently in the database. Example: the following set of 14 books:

0-12-333433-3, On Liberty, $25.00
0-103-45678-9, Iliad, $25.00
0-91-335678-7, Faerie Queene, $15.00
0-99-999999-9, Emma, $20.00
1-22-233700-0, Visual Basic, $25.00
1-1111-1111-1, C++, $29.95
0-91-045678-5, Hamlet, $20.00
0-555-55555-9, Macbeth, $12.00
0-99-777777-7, King Lear, $49.00
0-123-45678-0, Ulysses, $34.00
0-12-345678-9, Jane Eyre, $49.00
0-11-345678-9, Moby Dick, $49.00
0-321-32132-1, Balloon, $34.00
0-55-123456-9, Main Street, $22.95


Superkey

A set of attributes for an entity class that serves to uniquely identify an entity from among all possible entities in that entity class. Example: the set {Title, ISBN} for the Books entity class.


Key

A minimal superkey; that is, a key with the property that, if we remove an attribute, the resulting set is no longer a superkey. Example: the set {ISBN} for the Books entity class.


Table

A rectangular array of attribute values whose columns hold the attribute values for a given attribute and whose rows hold the attribute values for a given entity. Tables are used to implement entity sets. Example: the BOOKS table shown earlier in Table 3.1.


Table Scheme

The set of all attribute names for an entity class. Example:

{ISBN,Title,Price}

Since this is the table scheme for the entity class Books, we can use the notation Books (ISBN,Title,Price).


Relational Database

A finite collection of tables that provides an implementation of an E/R database model.


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