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Using Host Variables
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The embedded SQL programs in the previous figures don t provide any real interaction between the programming statements and the embedded SQL statements. In most applications, you will want to use the value of one or more program variables in the embedded SQL statements. For example, suppose you wanted to write a program to adjust all sales quotas up or down by some dollar amount. The program should prompt the user for the amount and then use an embedded UPDATE statement to change the QUOTA column in the SALESREPS table. Embedded SQL supports this capability through the use of host variables. A host variable is a program variable declared in the host language (for example, a COBOL or C variable) that is referenced in an embedded SQL statement. To identify the host variable, the variable name is prefixed by a colon (:) when it appears in an embedded SQL statement. The colon allows the precompiler to distinguish easily between host variables and database objects (such as tables or columns) that may have the same name.
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. . . exec sql whenever sqlerror goto error1; exec sql delete from salesreps where quota < 150000; exec sql delete from customers where credit_limit < 20000; exec sql whenever sqlerror continue; exec sql update salesreps set quota = quota * 1.05; exec sql whenever sqlerror goto error2; exec sql insert into salesreps (empl_num, name, quota) values (116, 'Jan Hamilton', 100000.00); . . . error1: printf("SQL DELETE error: %dl\n", sqlca.sqlcode); exit(); error2: printf("SQL INSERT error: %ld\n", sqlca.sqlcode); exit(); . . .
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Figure 17-16.
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Figure 17-17 shows a C program that implements the quota adjustment application using a host variable. The program prompts the user for the adjustment amount and stores the entered value in the variable named amount. This host variable is referenced
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in the embedded UPDATE statement. Conceptually, when the UPDATE statement is executed, the value of the amount variable is obtained, and that value is substituted for the host variable in the SQL statement. For example, if you enter the amount 500 in response to the prompt, the DBMS effectively executes this UPDATE statement:
exec sql update salesreps set quota = quota + 500;
A host variable can appear in an embedded SQL statement wherever a constant can appear. In particular, a host variable can be used in an assignment expression:
exec sql update salesreps set quota = quota + :amount;
A host variable can appear in a search condition:
exec sql delete from salesreps where quota < :amount;
A host variable can also be used in the VALUES clause of an INSERT statement:
exec sql insert into salesreps (empl_num, name, quota) values (116, 'Bill Roberts', :amount);
In each case, note that the host variable is part of the program s input to the DBMS; it forms part of the SQL statement submitted to the DBMS for execution. Later, in the section Data Retrieval in Embedded SQL, you will see how host variables are also used to receive output from the DBMS; they receive query results returned from the DBMS to the program. Note that a host variable cannot be used instead of a SQL identifier. This attempted use of the host variable colname is illegal:
char *colname = "quota"; PROGRAMMING WITH SQL exec sql insert into salesreps (empl_num, name, :colname) values (116, 'Bill Roberts', 0.00);
Declaring Host Variables
When you use a host variable in an embedded SQL statement, you must declare the variable using the normal method for declaring variables in the host programming language. For example, in Figure 17-17, the host variable amount is declared using the
SQL: The Complete Reference
main() { exec sql exec sql float exec sql
include sqlca; begin declare section; amount; end declare section;
/* amount (from user) */
/* Prompt the user for the amount of quota increase/decrease */ printf("Raise/lower quotas by how much:"); scanf("%f", &amount); /* Update the QUOTA column in the SALESREPS table */ exec sql update salesreps set quota = quota + :amount; /* Check results of statement execution */ if (sqlqa.sqlcode != 0) printf("Error during update.\n"); else printf("Update successful.\n"); exit(); }
Figure 17-17.
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