oracle資料庫計算兩個時間型別欄位值的時間差，並轉換為合適的時間格式顯示（按時分秒展示）
1.背景
資料庫表名為tablename。其中兩個欄位為startdate(開始時間)，closedate(結束時間)。
2.需求
建立試圖，查詢出間隔時間並顯示為自定義的格式。
3.sql語句
select t.*, round(to_number(t.closedate-t.startdate)*24*60*60)||'秒' "線上時長（秒）",
(
decode((trunc(to_number(t.closedate-t.startdate)*24)),null,null,0,null,trunc(to_number(t.closedate-t.startdate)*24)||'時')
||
decode((trunc(mod((to_number(t.closedate-t.startdate)*24*60),60))),null,null,0,null,trunc(mod((to_number(t.closedate-t.startdate)*24*60),60))||'分')
||
decode((trunc(mod((to_number(t.closedate-t.startdate)*24*60*60),60))),null,'無',trunc(mod((to_number(t.closedate-t.startdate)*24*60*60),60))||'秒')) "登入時長（顯示為 **時**分**秒 的形式）",
(select count(*) from tablename b where b.userbiscode=t.userbiscode and to_char(b.startdate,'yyyy-mm-dd')=to_char(sysdate,'yyyy-mm-dd')) "今日登入次數",
(select closedate from
(select a.closedate,rownum as rn from
(select closedate from tablename where to_char(startdate,'yyyy-mm-dd')=to_char(sysdate,'yyyy-mm-dd') order by startdate desc) a)
where rn<2) "最後登入時間"
from tablename t
WHERE to_char(t.startdate,'yyyy-mm-dd')=to_char(sysdate,'yyyy-mm-dd') order by t.startdate desc;