SQL/MP Programming Manual for C

Dynamic SQL Operations
HP NonStop SQL/MP Programming Manual for C—429847-008
10-12
Input Parameters and Output Variables
statement with an input SQLDA structure to get information about the input parameters
and obtain pointers to the input values.
NonStop SQL/MP returns data to a program through output variables. Output variables
are user-specified areas in the program. Output variables can be host variables or
individual data buffers to which the program (through the SQLDA structure) contains
pointers. Output variables usually contain columns returned from a SELECT operation.
A program uses the DESCRIBE statement to get information about the output
variables.
This sequence shows a typical context for input parameters and output variables in
dynamic SQL. If you know in advance which columns are likely to be selected, you can
use this sequence:
strcpy ( hostvar, "SELECT empnum, salary FROM =employee \
WHERE salary > ?sal"); /* input parameter sal */
/* Blank pad the statement buffer, dynamically compile */
/* the statement, describe its variables, prompt the */
/* user and read in the value for sal, declare and */
/* open a cursor for the statement. */
...
EXEC SQL
FETCH cursor INTO :enum, :sal; /* output variables */
/* :enum and :sal */
If you do not know in advance which columns to select, you can send the output values
to data buffers the program allocated earlier and to which the program set up pointers.
The pointers are in the SQLDA structure. In this case, the FETCH statement would
look like this:
EXEC SQL
FETCH cursor
USING DESCRIPTOR :sqlda; /* SQLDA contains pointers */
/* to output data buffers. */
Internally, SQL execution is the same for both scenarios.
Using a Parameter List
To ensure a one-to-one correspondence between a parameter list and the host
variables you use to supply values for the parameters, use unnamed parameters. If
duplicate parameter names appear in a statement, the names require a value for only
the first occurrence, and the duplicate occurrences receive the same value.
For example, suppose this UPDATE statement is stored in the host variable named
update_statement:
UPDATE table SET col1 = ?a, col2= ?a, col3 = ?b