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AN OVERVIEW OF SQL
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SQL and Portability
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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 holes in the SQL-89 standard and the current differences between SQL dialects are significant enough that an application must always be modified when moved from one SQL database to
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Figure 3-1.
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another. These differences, many of which were eliminated by the SQL2 standard but have not yet been implemented in commercial products, include: I Error codes. The SQL-89 standard does not specify the error codes to be returned when SQL detects an error, and all of the commercial implementations use their own set of error codes. The SQL2 standard specifies standard error codes. I Data types. The SQL-89 standard defines a minimal set of data types, but it omits some of the most popular and useful types, such as variable-length character strings, dates and times, and money data. The SQL2 standard addresses these, but not new data types such as graphics and multimedia objects. I System tables. The SQL-89 standard is silent about the system tables that provide information regarding the structure of the database itself. Each vendor has its own structure for these tables, and even IBM s four SQL implementations differ from one another. The tables are standardized in SQL2, but only at the higher levels of compliance, which are not yet provided by most vendors. I Interactive SQL. The standard specifies only the programmatic SQL used by an application program, not interactive SQL. For example, the SELECT statement used to query the database in interactive SQL is absent from the SQL-89 standard. Again, the SQL2 standard addressed this issue, but long after all of the major DBMS vendors had well-established interactive SQL capabilities. I Programmatic interface. The original standard specifies an abstract technique for using SQL from within an applications program written in COBOL, C, FORTRAN, and other programming languages. No commercial SQL product uses this technique, and there is considerable variation in the actual programmatic interfaces used. The SQL2 standard specifies an embedded SQL interface for popular programming languages but not a call-level interface. The 1995 SQL/CLI standard finally addressed programmatic SQL access, but not before commercial DBMS products had popularized proprietary interfaces and deeply embedded them in hundreds of thousands of user applications and application packages. I Dynamic SQL. The SQL-89 standard does not include the features required to develop general-purpose database front-ends, such as query tools and report writers. These features, known as dynamic SQL, are found in virtually all SQL database systems, but they vary significantly from product to product. SQL2 includes a standard for dynamic SQL. But with hundreds of thousands of existing applications dependent on backward compatibility, DBMS vendors have not implemented it. I Semantic differences. Because the standards specify certain details as implementor-defined, it s possible to run the same query against two different conforming SQL implementations and produce two different sets of query results. These differences occur in the handling of NULL values, column functions, and duplicate row elimination.
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SQL in Perspective
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I Collating sequences. The SQL-89 standard does not address the collating (sorting) sequence of characters stored in the database. The results of a sorted query will be different if the query is run on a personal computer (with ASCII characters) and a mainframe (with EBCDIC characters). The SQL2 standard includes an elaborate specification for how a program or a user can request a specific collating sequence, but it is an advanced-level feature that is not typically supported in commercial products. I Database structure. The SQL-89 standard specifies the SQL language to be used once a particular database has been opened and is ready for processing. The details of database naming and how the initial connection to the database is established vary widely and are not portable. The SQL2 standard creates more uniformity but cannot completely mask these details. Despite these differences, commercial database tools boasting portability across several different brands of SQL databases began to emerge in the early 1990s. In every case, however, the tools require a special adapter for each supported DBMS, which generates the appropriate SQL dialect, handles data type conversion, translates error codes, and so on. Transparent portability across different DBMS brands based on standard SQL is the major goal of SQL2 and ODBC, and significant progress has been made. Today, virtually all programs that support multiple databases include specific drivers for communicating with each of the major DBMS brands, and usually include an ODBC driver for accessing the others.
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