The primary feature of a relational database is its primary key, which is a unique identifier assigned to every record in a table. An example of a good primary key is a registration number. It makes every record unique, facilitating the storage of data in multiple tables, and every table in a relational database must have a primary key field.
Another key feature of relational databases is their ability to hold data over multiple tables. This feature overcomes the limitations of simple flat file databases that can only have one table. The database records stored in a table are linked to records in other tables by the primary key. The primary key can join the table in a one-to-one relationship, one-to-many relationship or many-to-many relationship.
Relational databases enable users to delete, update, read and create data entries in the database tables. This is accomplished though structured query language, or SQL, which is based on relational algebraic principles. SQL also enable users to manipulate and query data in a relational database.
Relational tables follow various integrity rules that ensure the data stored in them is always accessible and accurate. The rules coupled with SQL enable users to easily enforce transaction and concurrency controls, thus guaranteeing data integrity. The relational database concept was established by Edgar F. Codd in 1970.