Table, Entity, Record, Field
A database is a collection of information organized in such a way that a computer program can select desired pieces of data in a fast manner. A database can be thought of as an electronic filing system. Databases that hold data in table form, and represent relationships between tables are considered relational databases. The term relational comes from a table’s more formal name, relation. Essentially all databases now are relational databases, but were not in the past.
The most important thing about a relational database is a database object called a table. See Video 1 for an example and more information. Tables, combined with data, and the relationships among rows in tables make up the core of the database. (For an example of a table - click) The data within a table does not necessarily have to be stored in the database. Views are relational tables but their data are calculated at query time. A table organizes data grouped in rows and columns, which is very similar to a spreadsheet. The table is divided into horizontal, left to right, rows, and vertical, top to bottom, columns of data. A table has a specified number of columns, but can have an unlimited number of rows. Besides the data rows, tables usually contain some meta-information associated with them, such as constraints on the table or on the values within particular columns. See Video 2 for more information on creating tables in Microsoft Access
Within the relational database, each table represents a single entity. An entity is usually some aspect of the real world that can be distinguished from other aspects of the real world. An entity is anything that users of a database want to track or store data about, such as a person, place, or event. For example, in a database with an accounting application, data could be stored about customers, products, suppliers, invoices, and payments. Data relating to materials and production steps would be included as well if the company made the product. An entity set refers to a particular grouping of data. An entity is simply one example of a particular entity set. For example, employee John Smith, is but one member of the "employee" entity set.
An entity has to be something that can be uniquely identified. It is required that every entity have a primary key, a unique identifier that uniquely distinguishes an instance of the entity. There can not be repeat instances where any two or more entities have the same value for the primary key. Though natural attributes can be good primary keys at times, surrogate keys are often used instead. A surrogate key is an artifical attribute assigned to an entity, such as order ID or student ID, even though it has no inherent meaning because it can uniquely identify the entity.
To track and store data on an entity, table columns are grouped into rows, or records. Each bit of data is represented in a field, and fields are combined to form a record, or row. Every record contains data relating to the tables columns. Each record, as an instance of an entity, is assigned a unique key; the primary key of the table. Another word for columns are fields, which are groups of bytes in a
Each column represents one type of information, known as an attribute. An attribute is a characteristic of an entity that we want to store and record. Each entity has its own set of attributes that
refer to the measurement of interest. In a relational database, an entity attribute is associated with a column in a table. An example of attributes of the entity Salesperson would be SalespersonName, SalespersonPhone, SalespersonEmail, and so on. Attributes are represented on the ER Diagram by
ellipses. The name of the attribute is shown inside the ellipse and a line is drawn from the attribute to its entity. For more information on Entity Relationships, see Video 3 (left).