The Beginnings of SQL Oracle Tips by Burleson Consulting

In 1970, Dr. Edgar Codd of IBM and Chris Date developed a relational model for data storage. In the model, data would be stored in simple linear structures called “relations” or “tables.” One of the best improvements of the relational model over its predecessors was its simplicity. Rather than requiring users to know dozens of navigational data manipulation language (DML) commands, the relational model introduced a declarative language called SQL to simplify data access and manipulation.

In Codd and Date’s model, the tables are represented as two-dimensional arrays of “rows” and “columns.” Rows were called “tuples” (rhymes with “couples”), and columns were called “attributes.” A table will always have a field or several fields that make a “primary key” for a table. In their relational database model, the tables are independent, unlike in hierarchical and network models, where they are pointer-connected.

The relational database model offered the following improvements over the existing hierarchical and network databases:

•  Simplicity The concept of tables with rows and columns is extremely simple and easy to understand. End users have a simple data model. Complex network diagrams used with the hierarchical and network databases are not used with a relational database.

• Data independence Data independence is the ability to  modify data structures (in this case, tables) without affecting existing programs. Much of this ability comes because tables are not hard-linked to one another. Columns can be added to tables, tables can be added to the database, and new data relationships can be added with little or no restructuring of the tables. A relational database provides a much higher degree of data independence than do hierarchical and network databases.

• Declarative Data Access The SQL user specifies what data is  wanted, and then the embedded SQL, a procedural language, determines how to get the data. In relational database access, the user tells the system the conditions for the retrieval of data. The system then gets the data that meets the selection conditions in the SQL statements. The database navigation is hidden from the end user or programmer, unlike in a CODASYL DML language, where the programmer had to know the details of the access path.

In the marketplace, the declarative data access capability was far more interesting than the internal storage components of the relational database, and SQL became synonymous with the relational model.
 

This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.