natural join 为何物

natural join 的存在看似多余 因为它不够严谨，不够灵活，自己匹配相同字段的相同内容，近而进行连接

create table temp_zhangxb

(

   ABC varchar2(30)

);

create table temp_zhangxb1

(

   ABC varchar2(30)

);

create table temp_zhangxb2

(

   ABCD varchar2(30),

   ABC varchar2(30)

);

insert into temp_zhangxb (ABC)

values ('1');

insert into temp_zhangxb (ABC)

values ('2');

insert into temp_zhangxb1 (ABC)

values ('2');

insert into temp_zhangxb1 (ABC)

values ('3');

insert into temp_zhangxb2 (ABCD, ABC)

values ('2', '2');

insert into temp_zhangxb2 (ABCD, ABC)

values ('3', '3');

SELECT ABC FROM temp_zhangxb NATURAL JOIN temp_zhangxb1;

SELECT ABC,ABCD FROM temp_zhangxb NATURAL JOIN temp_zhangxb2;