Using BASED ON to Declare Variables

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InterBase supports a declarative clause, BASED ON, for creating C language character variables based on column definitions in a database. Using BASED ON ensures that the resulting host-language variable is large enough to hold the maximum number of characters in a CHAR or VARCHAR database column, plus an extra byte for the null-terminating character expected by most C string functions.

BASED ON uses the following syntax: 

BASED ON <dbcolumn> hostvar;

For example, the following statements declare two host variables, fname, and lname, based on two column definitions, FIRSTNAME, and LASTNAME, in an employee database: 

BASED ON EMP.FIRSTNAME fname;
BASED ON EMP.LASTNAME lname;

Embedded in a C or C++ program, these statements generate the following host- variable declarations during preprocessing: 

char fname[26];
char lname[26];

To use BASED ON, follow these steps:

  1. Use SET DATABASE to specify the database from which column definitions are to be drawn.
  2. Use CONNECT to attach to the database.
  3. Declare a section with BEGIN DECLARE SECTION.
  4. Use the BASED ON statement to declare a string variable of the appropriate type.

The following statements show the previous BASED ON declarations in context: 

EXEC SQL
SET DATABASE EMP = 'employee.ib';
EXEC SQL
CONNECT EMP;
EXEC SQL
BEGIN DECLARE SECTION;
int empno;
BASED ON EMP.FIRSTNAME fname;
BASED ON EMP.LASTNAME lname;
EXEC SQL
END DECLARE SECTION;

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