Forum OpenACS Q&A: Response to SQL statement return 0 row

I find that proc database_to_tcl_string expects the SQL to be a select statement that returns exactly one row and returns that row as an ns_set.

if
{
    set selection [ns_db 1row $db $sql]
}
else
{
    set selection [ns_db 1row $db [db_sql_prep $sql]]
}

there is why when PG return 0 row, the proc can't handle it.

If I am wrong, please correct me.

I change proc database_to_tcl_string into this format:

if
{
    set selection [ns_db 0or1row $db $sql]
}
else
{
    set selection [ns_db 0or1row $db [db_sql_prep $sql]]
}

However, it comes out with another error:

Error: invalid set id: ""
invalid set id: ""
    while executing
"ns_set value $selection 0"
    invoked from within
"return [ns_set value $selection 0]..."

Can I know what is the problem