unable to insert column using postgresql and C

Dear Sir/Madam,

I am a newbie in Postgresql and using C program to access it.

The following program (below; also in the attachment) reads an existing
table "people" having four columns (id, lastname, firstname, phonenumber)
and adds a new row (5, XXX, YYY, 7633839276) and a new column (TRUE).

After the proper building and linking and compiling, the program is
successfully inserting the row but its unable to insert the column and I
get the following output:

$ ./test
Successfully inserted value in Table.....
We did not get any data!

Can you please tell me where I am going wrong..

Thanks,
Madhurima

Program:

#include <stdio.h>
#include <stdlib.h>
#include <libpq-fe.h>
#include <string.h>

int main()
{
PGconn *conn;
PGresult *res;
int rec_count;
int row;
int col;

conn = PQconnectdb("dbname=test host=localhost user=abc password=xyz");

if(PQstatus(conn) == CONNECTION_BAD) {
puts("We were unable to connect to the database");
exit(0);
}

res = PQexec(conn,"INSERT INTO people VALUES (5, 'XXX', 'YYY',
'7633839276');");
if(PQresultStatus(res) != PGRES_COMMAND_OK) {
fprintf(stderr, "Insertion Failed: %s", PQerrorMessage(conn));
PQclear(res);
}
else
printf("Successfully inserted value in Table..... \n");

res = PQexec(conn,"update people set phonenumber=\'5055559999\' where
id=3");
res = PQexec(conn,"select lastname,firstname,phonenumber from people
order by id");
res = PQexec(conn, "ALTER TABLE people ADD comment VARCHAR(100);");
res = PQexec(conn, "EXEC('UPDATE people SET comment = ''TRUE'';');");

if(PQresultStatus(res) != PGRES_TUPLES_OK) {
puts("We did not get any data!");
exit(0);
}

rec_count = PQntuples(res);

printf("We received %d records.\n", rec_count);
puts("==========================");

for(row=0; row<rec_count; row++) {
for(col=0; col<3; col++) {
printf("%s\t", PQgetvalue(res, row, col));
}
puts("");
}

puts("==========================");

PQclear(res);

PQfinish(conn);

return 0;
}