conditional IF statements in postgresql

Hi,

I am writing a C program to access a PostgreSQL database, where

I add a column if it doesn't exists in the table

or, update the column, if the column already exits.

Please suggest how to work with the conditional statements.

Thanks!

N.B. I wrote the following:

res = PQexec(conn, "IF COL_LENGTH('protein_sequence','comment') IS NULL");
PQclear(res);
if(res)
{
res = PQexec(conn, "ALTER TABLE protein_sequence ADD comment
VARCHAR(500)");
PQclear(res);
}
else
{
res = PQexec(conn, "UPDATE TABLE protein_sequence ADD comment
VARCHAR(500)");
PQclear(res);
}

Is the code logically correct??