Glossary of Terms and Definitions
May 9 2003, 09:56 PM
Joined: 31-January 00
From: South Carolina, USA
It appears that many new users (and maybe an old one or two) may not know all of the terms used in many of the answers given. I thought I might try to post my definition of a few of these. They are in no particular order (only as I think of them).
ENTITY: n. A physical or virtual object that exists as a unique and discreet unit. It cannot be broken down or have anything removed from it without losing the integrity of the entity. (for example, an automobile that comes off the assembly line is an entity (it can be uniquely identified and is not whole if any part of it is removed), it is itself made up of other entities (all of it's myriad parts that are sometimes made up of other parts) each of which is an entity unto itself (it would lose integrity of what defines it as a part if anything were removed). A table should represent a single entity, so a table is in fact the entity.
ATTRIBUTE: n. A quality or characteristic inherent in or ascribed to someone or something. Another common name for an attribute is DESCRIPTOR. An attribute describes some aspect of an entity. Usually, it is one or more attributes that fully define and characterise the uniqueness of an entity. When this happens, this attribute (or combination of attributes) is said to be a "natural key".
KEY: n. The key represents the entity. In fact, it can be said that the key is the entity (or at least the database's representation of the entity)
CANDIDATE KEY: n. One of several possible attributes or combinations of attributes which can be used to uniquely identify a body of information (entity). Generally speaking, candidate keys should rarely be used as the PRIMARY KEY.
PRIMARY KEY: n The CANDIDATE KEY selected that uniquely identifies a body of information (entity). In Access (or any other RDBMS) for that matter it is usually desirable to use a single field as the primary key. This makes it much easier to link related tables to the entity. It makes it much easier to ensure uniqueness and (with Access) facilitates the updating and deleting of records in related tables.
RELATIONSHIP: n. The condition or fact of being related; connection or association. In database terms this is usually relegated to dependency. Access only directly supports one-to-one and one-to-many relationships. But usually it can always be said that one table (entity) is the "parent" of another table (entity).
FOREIGN KEY: n. Attributes contained within a database record (or other collection of related data items) which uniquely identify another record, but not the one within which they are contained. A foreign key is a reference from one record to another. In Access you create a foreign key by defining a field in the "child" table that will hold the value of the primary key of another table. This is how a related record in one table "knows" which parent record it is related to. All foreign keys must be the same datatype as the parent primary key.
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