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Before the introduction of database management systems, all data permanently stored on a computer system, such as payroll and accounting records, was stored in individual files. A file management system, usually provided by the computer manufacturer as part of the computer s operating system, kept track of the names and locations of the files. The file management system basically had no data model; it knew nothing about the internal contents of files. To the file management system, a file containing a word processing document and a file containing payroll data appeared the same. Knowledge about the contents of a file which data it contained and how the data was organized was embedded in the application programs that used the file, as shown in Figure 4-1. In this payroll application, each of the COBOL programs that processed the employee master file contained a file description (FD) that described the layout of the data in the file. If the structure of the data changed for example, if an additional item of data was to be stored for each employee every program that accessed the file had to be modified. As the number of files and programs grew over time, more and more of a data-processing department s effort went into maintaining existing applications rather than developing new ones. The problems of maintaining large file-based systems led in the late 1960s to the development of database management systems. The idea behind these systems was simple:

As its name implies, the DECLARE CURSOR statement is a declaration of the cursor In most SQL implementations, including the IBM SQL products, this statement is a directive for the SQL precompiler; it is not an executable statement, and the precompiler does not produce any code for it Like all declarations, the DECLARE CURSOR statement must physically appear in the program before any statements that reference the cursor that it declares Most SQL implementations treat the cursor name as a global name that can be referenced inside any procedures, functions, or subroutines that appear after the DECLARE CURSOR statement It s worth noting that not all SQL implementations treat the DECLARE CURSOR statement strictly as a declarative statement, and this can lead to subtle problems.

4:

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Some SQL precompilers actually generate code for the DECLARE CURSOR statement (either host language declarations or calls to the DBMS, or both), giving it some of the qualities of an executable statement For these precompilers, the DECLARE CURSOR statement must not only physically precede the OPEN, FETCH, and CLOSE statements that reference its cursor, but it also must sometimes precede these statements in the flow of execution, or be placed in the same block as the other statements In general, you can avoid problems with the DECLARE CURSOR statement by following these guidelines: Place the DECLARE CURSOR statement right before the OPEN statement for the cursor This placement ensures the correct physical statement sequence; it puts the DECLARE CURSOR and the OPEN statements in the same block; and it ensures that the flow of control passes through the DECLARE CURSOR statement, if necessary.

Figure 4-1.

It also helps to document just what query is being requested by the OPEN statement Make sure that the FETCH and CLOSE statements for the cursor follow the OPEN statement physically as well as in the flow of control..

take the definition of a file s content and structure out of the individual programs, and store it, together with the data, in a database. Using the information in the database, the DBMS that controlled it could take a much more active role in managing both the data and changes to the database structure.

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The OPEN statement, shown in Figure 17-25, conceptually opens the table of query results for access by the application program. In practice, the OPEN statement actually causes the DBMS to process the query, or at least to begin processing it. The OPEN statement thus causes the DBMS to perform the same work as an interactive SELECT statement, stopping just short of the point where it produces the first row of query results. The single parameter of the OPEN statement is the name of the cursor to be opened. This cursor must have been previously declared by a DECLARE CURSOR statement. If the query associated with the cursor contains an error, the OPEN statement will produce a negative SQLCODE value. Most query-processing errors, such as a reference to an unknown table, an ambiguous column name, or an attempt to retrieve data from a table without the proper permission, will be reported as a result of the OPEN statement. In practice, very few errors occur during the subsequent FETCH statements.

The programming required to perform a dynamic query is more extensive than the programming for any other embedded SQL statement. However, the programming is typically more tedious than complex. Figure 18-9 shows a small query program that uses dynamic SQL to retrieve and display selected columns from a user-specified table. The callouts in the figure identify the eight steps in the preceding list.

FIGURE 17-25

main() { /* This is a simple general-purpose query program. It prompts the user for a table name, and then asks the user which columns of the table are to be included in the query. After the user s selections are complete, the program runs the requested query and displays the results. */ exec sql include sqlca; exec sql include sqlda; exec sql begin declare section; char stmtbuf[2001]; char querytbl[32]; char querycol[32]; exec sql end declare section;

Part V:

/* SQL text to be executed */ /* user-specified table */ /* user-specified column */

FIGURE 17-26

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