将字符串转换为指定日期格式的字符串.
to_char(to_date(t1.zzsj,'yyyy-MM-dd'),'yyyy-MM-dd')
to_char(to_date(t1.zzsj,'yyyy-MM-dd'),'yyyy-MM-dd')
substr(t1.zhsj,1,4)||'-'||substr(t1.zhsj,5,2)||'-'||substr(t1.zhsj,7,2) as zhsj,
上面方式获取日期时, 值为空时会出现'--'情况.