關於oracle中的去除null和空格的問題

(length(trim(cbkssj)) is null
這個語句應該只適用於oracle的奇怪情況,判斷length等於null?!

除了這個以外奉送一條滿強的sql文法,裡面有很多的or和and,經過這次的經驗,我發現如果寫錯了or和and會增加語句的難度.

已知:有兩個時間欄位,kssj和jssj,如果都為空白則算全年,其他即使大於kssj的,算以後的時間.小於jssj的算今年開始至結束時間的.

最後的語句:
 where ((length(trim(cbkssj)) is null and
                               (length(trim(cbjssj)) is null or
                               substr(trim(cbjssj), 1, 6) >= v_yf)) or
                               (substr(trim(cbkssj), 1, 6) <= v_yf and
                               (length(trim(cbjssj)) is null or
                               substr(trim(cbjssj), 1, 6) >= v_yf)))) t1,