Oracle将时分秒格式的字符串转换为秒进行计算

        最近的需求中一个日期格式为'时分秒'且该字段的数据类型是字符串，然后需要计算总的时间，于是想了想，在sql中这样处理可达到我想要的效果，代码如下    

 select (hours+minutes+seconds) AS total_seconds from(     select to_number(substr('00:12:20',0,2))*60*60 AS hours,        to_number(substr('00:12:20',4,2))*60 AS minutes,         to_number(substr('00:12:20',7,2)) AS seconds  from dual )