合并 union

1、兩個表列要一致

2、合并時進行了去重操作

select * from test1 union select * from test2;

select id,name from test1 union select id,name from test2;

左右連接

left join? on 兩個表的對應關系--以左表為基準，右邊進行連接

左表中全部展示，坐表中有右表沒有用null補全，左表中沒有右表中有的則刪除

right join? on 兩個表的對應關系--與左表連接相反

select * from test1 right join test2 on test1.id=test2.id;

1、select * from test1 right join test2 on test1.id=test2.id;

(select * from test1 left join test2 on test1.id=test2.id) union (select * from test1 right join test2 on test1.id=test2.id);