How to use dynamic sql...?? in ECPG.....

In C programming... with ECPG tool...
I have some problem!!
How to use dynamic sql cursor??

void main()
{
char sqlsentence[255];
int flag;

....... Db Connection.....

switch(flag) {
case 1: {
sprintf(sqlsentence,"select id , name from testa where id = 1");
break;
}
case 2: {
sprintf(sqlsentece," select tel, address from testb where key=22");
break;
}
}

EXEC SQL DECLARE cur1 CURSOR FOR :sqlsentence;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
==> ecpg pre compile error "parse error"!!!
==> How to use Dynamic sql with ECPG??

....... some process.....

}