A relational database at its simplest is a set of tables used for storing data. Each table has a unique name and may relate to one or more other tables in the database through common values.
A table in a database is a collection of rows and columns. Tables are also known as entities or relations.
A row contains data pertaining to a single item or record in a table. Rows are also known as records or tuples.
A column contains data representing a specific characteristic of the records in the table. Columns are also known as fields or attributes.
A relationship is a link between two tables (i.e, relations). Relationships make it possible to find data in one table that pertains to a specific record in another table.
Each of a table's columns has a defined datatype that specifies the type of data that can exist in that column. For example, the
FirstName column might be defined as
varchar(20), indicating that it can contain a string of up to 20 characters. Unfortunately, datatypes vary widely between databases.
Most tables have a column or group of columns that can be used to identify records. For example, an
Employees table might have a column called
EmployeeID that is unique for every row. This makes it easy to keep track of a record over time and to associate a record with records in other tables.
Foreign key columns are columns that link to primary key columns in other tables, thereby creating a relationship. For example, the
Customers table might have a foreign key column called
SalesRep that links to
EmployeeID, the primary key in the
Relational Database Management System
A Relational Database Management System (RDBMS), commonly (but incorrectly) called a database, is software for creating, manipulating, and administering a database. For simplicity, we will often refer to RDBMSs as databases.