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In contrast, the situation is quite different in programmatic SQL. Some of the early steps can be done at compile-time, when the programmer is developing the program. This leaves only the later steps to be done at runtime, when the program is executed by a user. When you use programmatic SQL, all DBMS products try to move as much processing as possible to compile-time, because once the final version of the program is developed, it may be executed thousands of times by users in a production application. In particular, the goal is to move optimization to compile-time if at all possible.
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The central idea of embedded SQL is to blend SQL language statements directly into a program written in a host programming language, such as C, Java, Pascal, COBOL, FORTRAN, PL/I, or Assembler. Embedded SQL uses the following techniques to embed the SQL statements: SQL statements are intermixed with statements of the host language in the source program. This embedded SQL source program is submitted to a SQL precompiler (or in the case of Java, a preprocessor), which processes the SQL statements. Variables of the host programming language can be referenced in the embedded SQL statements, allowing values calculated by the program to be used by the SQL statements. Program language variables are also used by the embedded SQL statements to receive the results of SQL queries, allowing the program to use and process the retrieved values. Special program variables are used to assign NULL values to database columns and to support the retrieval of NULL values from the database. Several new SQL statements that are unique to embedded SQL are added to the interactive SQL language, to provide for row-by-row processing of query results. Figure 17-2 shows a simple embedded SQL program, written in C. The program illustrates many, but not all, of the embedded SQL techniques. The program prompts the user for an office number, retrieves the city, region, sales, and target for the office, and displays them on the screen. Don t worry if the program appears strange, or if you can t understand all of the statements that it contains before reading the rest of this chapter. One of the disadvantages of the embedded SQL approach is that the source code for a program becomes an impure blend of two different languages, making the program hard to understand without training in both SQL and the programming language. Another disadvantage is that embedded SQL uses SQL language constructs not used in interactive SQL, such as the WHENEVER statement and the INTO clause of the SELECT statement both used in this program.
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FIGURE 17-2 A typical embedded SQL program
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main() { exec sql include sqlca; exec sql begin declare section; int officenum; char cityname[16]; char regionname[11]; float targetval; float salesval; exec sql end declare section;
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office number (from user) */ retrieved city name */ retrieved region name */ retrieved target */ retrieved sales */
/* Set up error processing */ exec sql whenever sqlerror goto query_error; exec sql whenever not found goto bad_number; /* Prompt the user for the employee number */ printf("Enter office number:"); scanf("%d", &officenum); /* Execute the SQL query */ exec sql select city, region, target, sales from offices where office = :officenum into :cityname, :regionname, :targetval, :salesval; /* Display the results printf("City: %s\n", printf("Region: %s\n", printf("Target: %f\n", printf("Sales: %f\n", exit(); */ cityname); regionname); targetval); salesval);
query_error: printf("SQL error: %ld\n", sqlca.sqlcode); exit(); bad_number: printf("Invalid office number.\n"); exit(); }
Developing an Embedded SQL Program
An embedded SQL program contains a mix of SQL and programming language statements, so it can t be submitted directly to a compiler for the programming language. Instead, it moves through a multistep development process, shown in Figure 17-3. The steps in the
17:
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