Introduction

History:

Dr. E. F. Codd published the paper, “A Relational Model of Data for Large Shared Data Banks”, in June 1970 in the Association of Computer Machinery (ACM) journal, Communications of the ACM. Codd’s model is now accepted as the definitive model for relational database management systems (RDBMS). The language, Structured English Query Language (SEQUEL) was developed by IBM Corporation, Inc., to use Codd’s model. SEQUEL later became SQL (still pronounced “sequel”). In 1979, Relational Software, Inc. (now Oracle) introduced the first commercially available implementation of SQL. Today, SQL is accepted as the standard RDBMS language.

How SQL Works:

The purpose of SQL is to provide an interface to a relational database such as Oracle Database, and all SQL statements are instructions to the database. In this SQL differs from general-purpose programming languages like C and BASIC. Among the features of SQL are the following:

  • It processes sets of data as groups rather than as individual units.
  • It provides automatic navigation to the data.
  • It uses statements that are complex and powerful individually, and that therefore stand alone. Flow-control statements were not part of SQL originally, but they are found in the recently accepted optional part of SQL, ISO/IEC 9075-5: 1996. Flow-control statements are commonly known as “persistent stored modules” (PSM), and the PL/SQL extension to Oracle SQL is similar to PSM.

SQL lets you work with data at the logical level. You need to be concerned with the implementation details only when you want to manipulate the data. For example, to retrieve a set of rows from a table, you define a condition used to filter the rows. All rows satisfying the condition are retrieved in a single step and can be passed as a unit to the user, to another SQL statement, or to an application. You need not deal with the rows one by one, nor do you have to worry about how they are physically stored or retrieved. All SQL statements use the optimizer, a part of Oracle Database that determines the most efficient means of accessing the specified data. Oracle also provides techniques that you can use to make the optimizer perform its job better.

SQL provides statements for a variety of tasks, including:

  • Querying data
  • Inserting, updating, and deleting rows in a table
  • Creating, replacing, altering, and dropping objects
  • Controlling access to the database and its objects
  • Guaranteeing database consistency and integrity

SQL unifies all of the preceding tasks in one consistent language.

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