Code controls the user login procedure

Chapter 16: Using SQL within Applications	337

Of course, as you may expect, an SQL statement that suddenly appears in the middle of a C program can present a challenge for a compiler that isn’t expecting it. For that reason, programs containing embedded SQL are usu-ally passed through a preprocessor before being compiled or interpreted. The EXEC SQL directive warns the preprocessor of the imminent appearance of SQL code.

As an example of embedded SQL, look at a program written in Oracle’s Pro*C version of the C language. The program, which accesses a company’s EMPLOYEE table, prompts the user for an employee name and then displays that employee’s salary and commission. It then prompts the user for new salary and commission data — and updates the employee table with it:

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You don’t have to be an expert in C to understand the essence of what this program is doing (and how it intends to do it). Here’s a rundown of the order in which the statements execute:

1. SQL declares host variables.

7. SQL updates the database with the new values.

8. C then displays an Operation complete message.

	The way the SQL part is passed to the database depends on the implemen-tation. You, as the application developer, don’t have to worry about any of this. The preprocessor takes care of it. You should be concerned about a few things, however, that do not appear in interactive SQL — things such as host variables and incompatible data types.