Database Systems Terms

DB schema

The description of a database.

Includes descriptions of the database structure and the constraints

Schema diagram

A diagrammatic display of (some aspects of) a database schema

Relation

A collection of a set of data consisting of tuples and attributes.

Another word for table

Tuple

Each row of a table

Attribute

Each column of a table

Degree

The number of attributes (columns) in a table

Cardinality

The number of tuples (rows) in a table

Mini world

Some part of the real world about which data is stores in a database.

Example: student grades and transcripts at a university

Entity

A thing, person, place, unit, object, or any item about which the data should be captured and stored in the form of tables.

Simple attribute

An attribute where each entity has a single atomic value

Composite attribute

An attribute that may be composed of several components

Multi-valued attribute

An attribute where an entity may have multiple values

Key attributes

An attribute of an entity type for which each entity must have a unique value

Candidate key

A single column or a group of multiple columns that uniquely identify each table record.

In other words, it is a key that can be a primary key.

There are no duplicates, it is unique to each record, and there are no null values

Primary key

Used to uniquely identify each table record.

Must be unique or must have a unique value, and cannot have a null value.

Foreign key

A single column or a group of multiple columns corresponding to the value of the primary key in another table

Alternate key

A secondary key after determining a primary key

Super key

A single column or a group of multiple columns that can uniquely identify each table record.

Similar to a candidate key, but it must meet uniqueness

Unique key

The same as a primary key, but can accept up to one null value