This is a helper class for building the WHERE part of an SQL string out of pieces. It has methods add, addClause, negateLast, and negate, which are all demonstrated in the example below.
Once you've built your WhereClause, it can be injected into a query with the %? or %l parameters.
$where = new WhereClause('and'); // create a WHERE statement of pieces joined by ANDs
$where->add('username=%s', 'Joe');
$where->add('password=%s', 'mypass');
// SELECT * FROM accounts WHERE (`username`='Joe') AND (`password`='mypass')
$results = DB::query("SELECT * FROM accounts WHERE %?", $where);
// DELETE FROM accounts WHERE (`username`='Joe') AND (`password`='mypass')
DB::delete('accounts', '%?', $where);
// UPDATE accounts SET is_admin=1 WHERE (`username`='Joe') AND (`password`='mypass')
DB::update('accounts', array('is_admin' => 1), '%?', $where);
$subclause = $where->addClause('or'); // add a sub-clause with ORs
$subclause->add('age=%i', 15);
$subclause->add('age=%i', 18);
$subclause->negateLast(); // negate the last thing added (age=18)
// SELECT * FROM accounts WHERE (`username`='Joe') AND (`password`='mypass') AND ((`age`=15) OR (NOT(`age`=18)))
$results = DB::query("SELECT * FROM accounts WHERE %?", $where);
$subclause->negate(); // negate this entire subclause
// SELECT * FROM accounts WHERE (`username`='Joe') AND (`password`='mypass') AND (NOT((`age`=15) OR (NOT(`age`=18))))
$results = DB::query("SELECT * FROM accounts WHERE %?", $where);