UPDATE syntax problem

Hi,

I'm developing a C++ script to update postgresql database records. The
user interacts with the script via an html form. That is, the user is
presented with the data from a particular record in an html form and
asked to update any number of fields in that record.

To perform a multiple column update in postgres one does:

UPDATE tablename
          SET column1 = 'blahblah',
              column2 = 'moreblahblah',
              column3 = 1234
          WHERE id = 555;

Here's an excerpt from the script:

    //sql statement
    string sql;

    sql = "UPDATE product SET ";

    if (param["new_catid"] != param["old_catid"])
    {
        sql += "catid = " + param["new_catid"] + ",";
    }
    else if (param["new_catname"] != param["old_catname"])
    {
        sql += "prodname = '" + param["new_catname"] + "',";
    }
    else if (param["new_unitcnt"] != param["old_unitcnt"])
    {
        sql += "unitcnt = '" + param["new_unitcnt"] + "',";
    }
    else if (param["new_wprice"] != param["old_wprice"])
    {
        sql += "wprice = " + param["new_wprice"];
    }

    sql += "WHERE prodid = '" + param["prodid"] + "'";

    int res = conn.Exec (sql.c_str()); //sql exec

Now the problem occurs when a user only wants to update certain columns,
which creates a syntax problem due to the comma (","). In other words
you cannot end a multiple column UPDATE statement with a comma followed by:

WHERE prodid = 'xyz';

Now I could probably solve this problem by building separate UPDATE
statements as such:

    if (param["new_catid"] != param["old_catid"])
    {
        sql = "UPDATE product SET ";
        sql += "catid = " + param["new_catid"];
        sql += "WHERE prodid = '" + param["prodid"] + "'";
        int res = conn.Exec (sql.c_str()); //sql exec
    }
    else if (param["new_catname"] != param["old_catname"])
    {
        sql = "UPDATE product SET ";
        sql += "prodname = '" + param["new_catname"] + "'";
        sql += "WHERE prodid = '" + param["prodid"] + "'";
        int res = conn.Exec (sql.c_str()); //sql exec
    }

This necessitates calling the database each time the if statement is
true. Is there perhaps a more efficient way of doing this? I'm assuming
that the fewer times you call the database the faster the program will run.

Thanks,

Mark Tessier