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main() { exec sql include sqlca; exec sql begin declare section; int repnum; struct { char name[16]; float quota; float sales; } repinfo; short rep_ind[3]; exec sql end declare section;
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/* employee number (from user) */ /* retrieved salesperson name */ /* retrieved quota */ /* retrieved sales */ /* null indicator array */
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/* Prompt the user for the employee number */ printf("Enter salesrep number: "); scanf("%d", &repnum); /* Execute the SQL query */ exec sql select name, quota, sales from salesreps where empl_num = :repnum into :repinfo :rep_ind; /* Display the retrieved data */ if (sqlca.sqlcode = = 0) { printf("Name: %s\n", repinfo.name); if (rep_ind[1] < 0) printf("quota is NULL\n"); else printf("Quota: %f\n", repinfo.quota); printf("Sales: %f\n", repinfo.sales); } else if (sqlca.sqlcode = = 100) printf("No salesperson with that employee number.\n"); else printf("SQL error: %ld\n", sqlca.sqlcode); exit(); }
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When the precompiler encounters a data structure reference in the INTO clause, it replaces the structure reference with a list of the individual variables in the structure, in the order they are declared within the structure. Thus, the number of items in the structure and their data types must correspond to the columns of query results. The use of data structures in the INTO clause is, in effect, a shortcut. It does not fundamentally change the way the INTO clause works. Support for the use of data structures as host variables varies widely among DBMS brands. It is also restricted to certain programming languages. DB2 supports C and PL/I structures but does not support COBOL or assembly language structures, for example.
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Host variables provide two-way communication between the program and the DBMS. In the program shown in Figure 17-21, the host variables repnum and repname illustrate the two different roles played by host variables: I The repnum host variable is an input host variable, used to pass data from the program to the DBMS. The program assigns a value to the variable before executing the embedded statement, and that value becomes part of the SELECT statement to be executed by the DBMS. The DBMS does nothing to alter the value of the variable. I The repname host variable is an output host variable, used to pass data back from the DBMS to the program. The DBMS assigns a value to this variable as it executes the embedded SELECT statement. After the statement has been executed, the program can use the resulting value. Input and output host variables are declared the same way and are specified using the same colon notation within an embedded SQL statement. However, it s often useful to think in terms of input and output host variables when you re actually coding an embedded SQL program. Input host variables can be used in any SQL statement where a constant can appear. Output host variables are used only with the singleton SELECT statement and with the FETCH statement, described in the next section of this chapter.
Multirow Queries
When a query produces an entire table of query results, embedded SQL must provide a way for the application program to process the query results one row at a time. Embedded SQL supports this capability by defining a new SQL concept, called a cursor, and adding several statements to the interactive SQL language. Here is an overview of
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SQL: The Complete Reference
embedded SQL techniques for multirow query processing and the new statements it requires: 1. The DECLARE CURSOR statement specifies the query to be performed and associates a cursor name with the query. 2. The OPEN statement asks the DBMS to start executing the query and generating query results. It positions the cursor before the first row of query results. 3. The FETCH statement advances the cursor to the first row of query results and retrieves its data into host variables for use by the application program. Subsequent FETCH statements move through the query results row by row, advancing the cursor to the next row of query results and retrieving its data into the host variables. 4. The CLOSE statement ends access to the query results and breaks the association between the cursor and the query results. Figure 17-23 shows a program that uses embedded SQL to perform a simple multirow query. The numbered callouts in the figure correspond to the numbers in the preceding steps. The program retrieves and displays, in alphabetical order, the name, quota, and year-to-date sales of each salesperson whose sales exceed quota. The interactive SQL query that prints this information is:
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