说起è¿ä¸¤ç§èæ¥æ¹å¼ï¼ä¸å®è¦æRight Joinè系起æ¥ã
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1ãLeft Joinï¼å·¦èæ¥ï¼
以左表为ä¸å¿ï¼è¿å左表ä¸ç¬¦åæ¡ä»¶çææè®°å½ä»¥åå³è¡¨ä¸èç»å段ç¸ççè®°å½ââå½å³è¡¨ä¸æ ç¸åºèæ¥è®°å½æ¶ï¼è¿å空å¼ã
2ãRight Joinï¼å³èæ¥ï¼
以å³è¡¨ä¸ºä¸å¿ï¼è¿åå³è¡¨ä¸ç¬¦åæ¡ä»¶çææè®°å½ä»¥å左表ä¸èç»å段ç¸ççè®°å½ââå½å·¦è¡¨ä¸æ ç¸åºèæ¥è®°å½æ¶ï¼è¿å空å¼ã
3ãInner Joinï¼çå¼è¿æ¥ï¼
è¿å两个表ä¸èç»å段ç¸ççè¡ã
äºã示ä¾ã
1ãæå ¥æµè¯è¡¨(test1,test2)
create table test1 --æµè¯è¡¨1
(id int not null,
value char(10) )
create table test2 --æµè¯è¡¨2
(id int not null,
value char(10) )
2ãæå ¥æ°æ®
--insert into test1
insert into test1
values (1,'testaa')
insert into test1
values (2,'testaa')
insert into test1
values (3,'testaa')
--insert into test2
insert into test2
values (1,'testaa2')
insert into test2
values (2,'testaa2')
insert into test2
values (4,'testaa2')
3ãæ¥è¯¢ç»ææ¯è¾ï¼éå¾ï¼
select * from test1 a left join test2 b on a.id = b.id
select * from test1 a right join test2 b on a.id = b.id
select * from test1 a inner join test2 b on a.id = b.id
4ãå é¤æµè¯è¡¨
drop table test1
drop table test2
我知道left join 我只是忘记join是不是left join的缩写了。你没说join是啥意思
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