create table testchar(
 v_date varchar2(20),
 d_date date
)

insert into testchar select '2018-07-20', date'2018-07-20' from dual;


// 这种将 tochar()字符串类型的字段v_date,就会报ORA-01722的错误
select to_char(v_date, 'yyyyMMdd') from testchar;