create table t_char (name char(20));
insert into t_char values('a');

create table t_varchar2(name varchar2(20));

insert into t_varchar2 select name from t_char;

select length(name) from t_varchar2;
--LENGTH(NAME)
------------
          20
--結果varchar2字段也占20位。空間浪費。如何改進?
 insert into t_varchar2 select trim(name) from t_char;

--在char到varchar2轉變中，去掉char的空各。