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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.


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


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


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.


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.


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:


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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