Our patent example takes two search arguments from the user, $query and $when , which we use in our SQL statement:

  <$null = "">
  <SQL ROW SKIP=$skip MAX=10
      "select pcountry, pnumber, pdate, id
       from patent
       where pabstract like $query and pdate >= $when
       order by pdate asc">

But what if the user only enters a text query, with a blank date? We need to remove the "and nbsp;pdate nbsp;>= nbsp;$when " part of the SQL clause. Similarly, if the text $query was blank, we'd need to remove "pabstract nbsp;like nbsp;$query " and leave in the other part.

The $null variable

It's a hassle to have to generate every permutation of the SQL where clause based on what the user entered. That's where the special variable $null comes in.

By setting $null , we automatically drop parts of the statement associated with unused parameters. If $null is set to contain any values (one empty string in our case), then any <SQL> parameters that match any of its values are dropped from the query, including the clause they're in. Unset (no-value) variables are considering empty strings for this comparision.

That's why we set <$null nbsp;= nbsp;""> just above the statement. Now, if the user leaves $when empty, it will match a value of $null , and its clause will be dropped. The SQL run by our <SQL> statement above would actually become:

  select pcountry, pnumber, pdate, id
  from patent
  where pabstract like $query
  order by pdate asc

which is what we want (no date search), without us having to re-work the SQL statement. Similarly, if $query is empty, the like clause is dropped. Had we set <$null nbsp;= nbsp;"" nbsp;"any"> , then not only empty fields but the word "any " entered by the user would cause the clause to drop. Pretty nifty huh?