conditional IF statements in postgresql

[Madhurima Das <madhurima.das@xxxxxxxxx>]

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??