The SQL language may be considered one of the major reasons for the commercial success of relational databases. Because it became a standard for relational databases, users were less concerned about migrating their database applications from other types of database systems—for example, network or hierarchical systems—to relational systems. This is because even if the users became dissatisfied with the particular relational DBMS product they were using, converting to another relational DBMS product was not expected to be too expensive and time-consuming because both systems followed the same language standards. In practice, of course, there are many differences between various commercial relational DBMS packages. However, if the user is diligent in using only those features that are part of the standard, and if both
relational systems faithfully support the standard, then conversion between the two systems should be much simplified. Another advantage of having such a standard is that users may write statements in a database application program that can access data stored in two or more relational DBMSs without having to change the database sublanguage (SQL) if both relational DBMSs support standard SQL.

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The name SQL is presently expanded as Structured Query Language. Originally, SQL was called SEQUEL (Structured English QUEry Language) and was designed and implemented at IBM Research as the interface for an experimental relational database system called SYSTEM R. SQL is now the standard language for commercial relational DBMSs. A joint effort by the American National Standards Institute (ANSI) and the International Standards Organization (ISO) has led to a standard version of SQL (ANSI 1986), called SQL-86 or SQL1. A revised and much expanded standard called SQL-92 (also referred to as SQL2) was subsequently developed. The next standard that is well-recognized is SQL:1999, which started out as SQL3. Two later updates to the standard are SQL:2003 and SQL:2006, which added XML features among other updates to the language. Another update in 2008 incorporated more object database features in SQL.

SQL is a comprehensive database language: It has statements for data definitions, queries, and updates. Hence, it is both a DDL and a DML. In addition, it has facilities for defining views on the database, for specifying security and authorization, for defining integrity constraints, and for specifying transaction controls. It also has rules for embedding SQL statements into a general-purpose programming language such as Java, COBOL, or C/C++.

The later SQL standards (starting with SQL:1999) are divided into a core specification plus specialized extensions. The core is supposed to be implemented by all RDBMS vendors that are SQL compliant. The extensions can be implemented as optional modules to be purchased independently for specific database applications such as data mining, spatial data, temporal data, data warehousing, online analytical
processing (OLAP), multimedia data, and so on.


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