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I Application program interface. In this approach, the program communicates with the DBMS through a set of function calls called an application program interface, or API. The program passes SQL statements to the DBMS through the API calls and uses API calls to retrieve query results. This approach does not require a special precompiler. The initial IBM SQL products used an embedded SQL approach, and most commercial SQL products adopted it in the 1980s. The original ANSI/ISO SQL standard specified only an awkward module language for programmatic SQL, but commercial SQL products continued to follow the IBM de facto standard. In 1989, the ANSI/ISO standard was extended to include a definition of how to embed SQL statements within the Ada, C, COBOL, FORTRAN, Pascal, and PL/I programming languages, this time following the IBM approach. The SQL2 standard continued this specification. In parallel with this evolution of embedded SQL, several DBMS vendors who were focused on minicomputer systems introduced callable database APIs in the 1980s. When the Sybase DBMS was introduced, it offered only a callable API. Microsoft s SQL Server, derived from the Sybase DBMS, also used the API approach exclusively. Soon after the debut of SQL Server, Microsoft introduced Open Database Connectivity (ODBC), another callable API. ODBC is roughly based on the SQL Server API, but with the additional goals of being database independent and permitting concurrent access to two or more different DBMS brands through a common API. More recently, Java Database Connectivity (JDBC) has emerged as an important API for accessing a relational database from within programs written in Java. With the growing popularity of callable APIs, the callable and embedded approaches are both in active use today. In general, programmers using older languages, such as COBOL and Assembler, will tend to favor the embedded SQL approach. Programmers using newer languages, such as C++ and Java, will tend to favor the callable API approach. The following table summarizes the programmatic interfaces offered by some of the leading SQL-based DBMS products:
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ODBC, JDBC
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Embedded SQL Language Support
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APL, Assembler, BASIC, COBOL, FORTRAN, Java, PL/I C, COBOL C
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Informix
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ODBC, JDBC
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Microsoft SQL Server DB Library (dblib), ODBC
SQL: The Complete Reference
DBMS
MySQL Oracle Sybase
Callable API
C-api (proprietary), ODBC, JDBC, Perl, PHP, Tcl Oracle Call Interface (OCI), ODBC, JDBC DB Library (dblib), ODBC, JDBC
Embedded SQL Language Support
None C, COBOL, FORTRAN, Pascal, PL/I, Java C, COBOL
The basic techniques of embedded SQL, called static SQL, are described in this chapter. Some advanced features of embedded SQL, called dynamic SQL, are discussed in 20. Callable SQL APIs, including the Sybase/SQL Server API, ODBC, and JDBC, are discussed in 21.
DBMS Statement Processing
To understand any of the programmatic SQL techniques, it helps to understand a little bit more about how the DBMS processes SQL statements. To process a SQL statement, the DBMS goes through a series of five steps, shown in Figure 17-1: 1. The DBMS begins by parsing the SQL statement. It breaks the statement up into individual words, makes sure that the statement has a valid verb, legal clauses, and so on. Syntax errors and misspellings can be detected in this step. 2. The DBMS validates the statement. It checks the statement against the system catalog. Do all the tables named in the statement exist in the database Do all of the columns exist, and are the column names unambiguous Does the user have the required privileges to execute the statement Semantic errors are detected in this step. 3. The DBMS optimizes the statement. It explores various ways to carry out the statement. Can an index be used to speed a search Should the DBMS first apply a search condition to Table A and then join it to Table B, or should it begin with the join and use the search condition afterward Can a sequential search through a table be avoided or reduced to a subset of the table After exploring alternatives, the DBMS chooses one of them. 4. The DBMS then generates an application plan for the statement. The application plan is a binary representation of the steps that are required to carry out the statement; it is the DBMS equivalent of executable code. 5. Finally, the DBMS carries out the statement by executing the application plan. Note that the steps in Figure 17-1 vary in the amount of database access they require and the amount of CPU time they take. Parsing a SQL statement does not require access
17:
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