When it comes to NULL values, PowerBuilder treats them a little different than other languages. Not only that, when null is added, compared to or concatenated with any variable the entire result will become null, but also when given a null parameter most PowerBuilder functions will simply return NULL and perform no action.

To avoid generating possible errors, the solution is to check for the null values and proceed accordingly.
A common example of such case is passing a null text value to the MessageBox function. The function will do simply nothing and the flow will continue with the next line of code.
So instead of:

    MessageBox("", ls_text);

one can wrapp the function like this:

    f_MessageBox(as_text)
    if (IsNull(as_text)) then
        MessageBox("", "null value received!");
    else
        MessageBox("", as_text);
    end if

On the other hand, in PowerBuilder unassigned variables/columns can take different values depending on their source. Table below shows how an unassigned PowerScript variable will be initialized:

When external stored procedures or functions need to be called with explicit null value parameters or for any other particular reason when a null value is needed for a variable leaving the value unassigned will generate wrong unexpected results. To proper specify a NULL value, use the SetNull function.

    int ln_location_id = 1; string ls_location_name;
    SetNull(ls_location_name);
    prepare sqlsa from "INSERT INTO locations values (?,?)";
    execute sqlsa using :ln_location_id, :ls_location_name;

Most SQL databases (eg PostgreSQL) do not use null in arithmetic comparisons.
For example following queries will return 0 records.

    SELECT * FROM locations WHERE "name" = null 
    SELECT * FROM locations WHERE "name" IN (null)

To get the desired result must use:

    SELECT * FROM locations WHERE "name" IS NULL

A common situation here is where null parameters are supplied to a stored procedure and the results are not what should expect. In this case must evaluate such variables and use a dynamic cursor to provide specific parameters to the function:

    string ls_query_where_clause, ls_query;
    DECLARE crs_stock DYNAMIC CURSOR FOR SQLSA;
    ls_query_where_clause = " WHERE article_id = '" + string(ll_article_id) + "'";
    if (isnull(ll_location_id)) then
        ls_query_where_clause += " AND location_id ISNULL";
    else
        ls_query_where_clause += " AND location_id = '" + string(ll_location_id) + "'";
    end if
    ls_query = "SELECT quantity, cost FROM inv_stock" + ls_query_where_clause;
    PREPARE SQLSA FROM :ls_query;
    sqlca.checksql(true, "");
    OPEN DYNAMIC crs_stock;
    sqlca.checksql(true, "");
    FETCH crs_stock INTO :ldbl_qty, :ldbl_cost;
    // read lines
    CLOSE crs_stock;