[solved] No 'MONTH(date_field)' in $this->db->where(); possible?

[eluser]Jan_1[/eluser]
Thank you, Armchair Samurai.
And sorry, I did'nt get it.

I've tried

Code:

$where1 = array('wg_id'=>$wg_id, 'YEAR(ks_datum)'=>'YEAR(CURDATE())',FALSE);
$where2 = array('wg_id'=>$wg_id, 'MONTH(ks_datum)'=>'MONTH(CURDATE())',FALSE);
$where3 = array('wg_id'=>$wg_id, 'MONTH(ks_datum)'=>'MONTH(SUBDATE(CURDATE(), INTERVAL 1 MONTH)',FALSE);

and the browser answered

A Database Error Occurred
Error Number: 1054
Unknown column '0' in 'where clause'
SELECT COUNT(ks_betrag) AS anzahl, SUM(ks_betrag) AS summe FROM (`ks_buchung`) WHERE `wg_id` = '34' AND YEAR(ks_datum) = 'YEAR(CURDATE())' AND `0` = 0

what I do understand (me idiot...).
So I tried

Code:

$this->db->select($select);
$this->db->where($where1, FALSE);  //<- also '($where1, Null, FALSE)'
$query = $this->db->get();
if ($query->num_rows() > 0) { $row = $query->row(); }
$summe_jahr = $row->summe;
$anzahl_jahr = $row->anzahl;

...
$this->db->where($where2, FALSE);
...
$this->db->where($where3, FALSE);
...

and the browser answered:

A Database Error Occurred
Error Number: 1064
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE `wg_id` = '34' AND MONTH(ks_datum) = 'MONTH(CURDATE())'' at line 2
SELECT COUNT(ks_betrag) AS anzahl, SUM(ks_betrag) AS summe WHERE `wg_id` = '34' AND MONTH(ks_datum) = 'MONTH(CURDATE())'

So, how do I have to place it?
I haven't find an example with array in the User Guide