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1.3 SQL Dialects
The constantly evolving nature of the SQL standard has given rise to a number of SQL dialects among the various vendors and platforms. These dialects most commonly evolved because the user community of a given database vendor required capabilities in the database before the ANSI committee created a standard. Occasionally, though, a new feature is introduced by the academic or research communities due to competitive pressures from competing technologies. For example, many database vendors are augmenting their current programmatic offerings with Java (as is the case with DB2, Oracle, and Sybase) or VBScript (as Microsoft is doing). In the future, programmers and developers will use these programming languages in concert with SQL to build SQL programs.
Many of these dialects include conditional processing capabilities (such as that controlled through IF . . . THEN statements), control-of-flow functions (such as WHILE loops), variables, and error handling. Because ANSI had not yet developed a standard for these important features at the time users began to demand them, RDBMS developers and vendors were free to create their own commands and syntax. In fact, some of the earliest vendors from the 1980s have variances in the most elementary commands, such as SELECT, because their implementations predate the standards. ANSI is now refining standards that address these inconsistencies.
Some of these dialects have introduced procedural commands to support the functionality of a much more complete programming language. For example, these procedural implementations contain error-handling commands, control-of-flow language, conditional commands, variable handling, arrays, and many other extensions. Although these are technically divergent procedural implementations, they are called dialects here. The SQL/PSM (Persistent Stored Module) package provides many features associated with programming stored procedures and incorporates many of the extensions offered by these dialects.
Some popular dialects of SQL include:
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