Can we use a host-variable as table name in SELECT query? - sql

Question

I'm experimenting with Pro*C code.
I have 3 tables emp, mgr, all; All 3 tables contain emp_id and emp_name. I tried the below code it is giving me error. Please let me know if it is possible?
const char table_name[3]={'emp','mgr','all'}
int counter = 0;
while(counter < 3)
{
. . .
EXEC SQL SELECT emp_name INTO :ename
From :table_name[counter++]
where emp_id=:emp_id;
}
Can we use variables for SELECT and FROM ?

Answer 1

This is called Dynamic SQL:
Use this info:
Dynamic SQL
While embedded SQL is fine for fixed applications, sometimes it is important for a program to dynamically create entire SQL statements.
With dynamic SQL, a statement stored in a string variable can be issued.
PREPARE turns a character string into a SQL statement, and EXECUTE executes that statement. Consider the following example.
char *s = "INSERT INTO emp VALUES(1234, 'jon', 3)";
EXEC SQL PREPARE q FROM :s;
EXEC SQL EXECUTE q;
Alternatively, PREPARE and EXECUTE may be combined into one statement:
char *s = "INSERT INTO emp VALUES(1234, 'jon', 3)";
EXEC SQL EXECUTE IMMEDIATE :s;
Source: http://infolab.stanford.edu/~ullman/fcdb/oracle/or-proc.html

Answer 2

No, we can't use a host-variable to supply a table-name to a static SQL statement in Pro*C.
To quote Oracle Pro*C Programmer's Guide for Oracle 11.2g:
You cannot use input host variables to supply SQL keywords or the names of database objects. [..] If you need to change database object names at runtime, use dynamic SQL. See also Chapter 13, "Oracle Dynamic SQL".
In Pro*C Oracle provides different methods to execute SQL statements. The main distinction is between static and dynamic methods. And for dynamic there are several sub-methods that allow different degrees of freedom.
A static SQL statement is not 100% static - you can use input/output host-variables in where clause expressions (as operands), to supply values in insert statements, as select targets, etc. But not to specify table names. This is 'too dynamic' for static embedded SQL.
Note that you can still use host-variables in your dynamically prepared SQL statements. This is recommended to avoid SQL injection issues and increase performance (when a statement is executed several times).
Example (uses Oracle Dynamic SQL method 3):
const char *table_name[3] = {"emp", "mgr", "all"};
unsigned table_name_size = 0;
unsigned i = 0;
for (i = 0; i<table_name_size; ++i) {
char stmt[128] = {0};
snprintf(stmt, 128,
"SELECT emp_name "
" FROM %s "
" WHERE "
" emp_id = :emp_id",
table_name[i]);
EXEC SQL PREPARE emp_stmt FROM :stmt;
// check sqlca.sqlcode ...
EXEC SQL DECLARE emp_cursor CURSOR FOR emp_stmt;
EXEC SQL OPEN emp_cursor USING :emp_id;
// check sqlca.sqlcode ...
EXEC SQL FETCH emp_cursor INTO :ename;
// check sqlca.sqlcode ...
EXEC SQL CLOSE emp_cursor;
// check sqlca.sqlcode ...
// ...
}