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ODBC and the SQL Access Group
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An important area of database technology not addressed by the earlier official standards is database interoperability the methods by which data can be exchanged among different databases, usually over a network. In 1989, a group of vendors formed the SQL Access Group to address this problem. The resulting SQL Access Group specification for Remote Database Access (RDA) was published in 1991. Unfortunately, the RDA specification was closely tied to the OSI networking protocols, which lost the networking battle to the Internet s TCP/IP suite, so RDA was never widely implemented. A second standard from the SQL Access Group had far more market impact. At Microsoft s urging and insistence, the SQL Access Group expanded its focus to include a call-level interface for SQL. Based on a draft from Microsoft, the resulting Call-Level Interface (CLI) specification was published in 1992. Microsoft s own Open Database Connectivity (ODBC) specification, based on the CLI standard, was published the same year. With the market power of Microsoft behind it, and the open standards blessing of the SQL Access Group, ODBC emerged as the de facto standard interface for PC access to SQL databases. Apple and Microsoft announced an agreement to support ODBC on Macintosh and Windows in the spring of 1993, giving ODBC industry standard status in both popular graphical user interface environments. ODBC implementations for UNIXbased systems soon followed. In 1995, the ODBC interface effectively became an ANSI/ ISO standard, with the publication of the SQL/Call-Level Interface (CLI) standard. Over the past decade, ODBC has continued to evolve, but at a slower pace. Microsoft still supports ODBC, but has focused major effort into building higher-level, more object-oriented
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interfaces for universal database access. However, ODBC still plays a major role providing portability across databases for major enterprise applications and for database tools. It s quite common for a database tool or an enterprise application to support specific drivers that optimize it for direct access to Oracle and DB2 and SQL Server using their proprietary call-level interfaces. The application will typically include an additional driver that uses ODBC as a way to support a broad range of other databases. Because so many applications and tools adopt this approach, nearly all database vendors offer ODBC access, sometimes as their primary call-level interface and sometimes as a supplement to a higher-performance, proprietary interface.
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The explosive popularity of the Internet drove the further development of database access standards to support the accompanying rise of the object-oriented Java programming language. Java eventually became the standard language for building Internet-delivered applications that ran on Java-based application servers. Sun Microsystems, the inventor of Java, led the effort to standardize the use of Java for application servers through the Java2 Enterprise Edition (J2EE) specification. J2EE included Java Database Connectivity (JDBC) as its standard for Java access to relational databases. Unlike database access for the C programming language, where proprietary call-language interfaces predated ODBC by many years, the JDBC standard was developed relatively early in the explosion of Java popularity. As a result, proprietary Java interfaces failed to emerge, and JDBC is the standard for SQL access from the Java language.
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The existence of published SQL standards has spawned quite a few exaggerated claims about SQL and applications portability. Diagrams such as the one in Figure 3-1 are frequently drawn to show how an application using SQL can work interchangeably with any SQL-based database management system. In fact, the differences between SQL dialects are significant enough that an application must often be modified when moved from one
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The SQL portability myth
SQL in Perspective
SQL database to another. Over time, the core of the language has become more standard and has broadened, but at the same time, new capabilities have been added by the database vendors, often with proprietary language extensions. Examples of areas where these differences arise include Data types The SQL standard has evolved to address an ever-broader set of data types, but vendors keep adding new ones. Even older data types can cause portability issues Oracle s NUMBER data type, for example, is the most widely used to represent numeric data in an Oracle database, and its peculiarities are completely unique to Oracle. Backward compatibility It s not uncommon for enterprise applications to still be in use 10 or 20 years after they were first written, long after the programmers who developed them are gone. These programs tend to become untouchable, since the detailed knowledge of how they work has often been lost. Large sections of these programs may depend on older, proprietary SQL features, and database vendors are forced to maintain backward compatibility with them or risk breaking the applications. This perpetuates dialect differences that inhibit portability. System tables The SQL standard addressed the system tables that provide information about the structure of the database itself starting with the SQL-92 standard. By this time, database vendors had built their own proprietary system table structures, and they have continued to evolve them, often containing useful information that goes well beyond the items specified in the standard. Applications that use these proprietary system tables are not portable. Programmatic interface The early SQL standard specified an abstract technique for using SQL from within an applications program written in COBOL, C, FORTRAN, and other programming languages, which was not widely adopted. The 1995 SQL/ CLI standard finally addressed programmatic SQL access, but by then, commercial DBMS products had popularized proprietary interfaces and deeply embedded them in hundreds of thousands of user applications and application packages. Although standard APIs are now widely supported, most database vendors still maintain proprietary interfaces that offer higher performance and richer functionality, with the side-effect of locking in applications. Semantic differences Because the standards specify certain details as implementer-defined, it s possible to run the same query against two different conforming SQL implementations and produce two different sets of query results. Examples of these differences can be found in areas like the handling of NULL values, column functions, and duplicate row elimination. Replication and data mirroring Many production databases contain tables that are replicated in two or more geographically separated databases, to provide high availability or disaster recovery, to spread out processing workloads, or to reduce network delays. The techniques for specifying and managing these replication schemes are proprietary to each database system, and attempts to standardize replication have been abandoned.
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