SQLFetch And SQL_NO_DATA

hi all~

struct user {
unsigned int uid;
unsigned int auth;
};

int funct(int idx) {
struct user usr;
SQLINTEGER ret[2];
SQLBindCol(hStmt, 1, SQL_C_ULONG, &usr.uid, 0, &ret[0]);
SQLBindCol(hStmt, 2, SQL_C_ULONG, &usr.auth, 0, &ret[1]);
if(SQLExecDirect(hStmt, (SQLCHAR*)"select uid,auth from my_tbl where idx=12", SQL_NTS) != SQL_SUCCESS) return -1;

if(SQLFetch(hStmt) == SQL_NO_DATA) return 0;

SQLCloseCursor(hStmt);

return 1;
}

sometimes SQLFetch returns SQL_NO_DATA while the my_tbl has a row whose idx is 12.

it causes critical problem in my system.
Though table has a row, why does the SQLFetch returns SQL_NO_DATA??