A表
id name
1 bai
2 zhang
3 li
B表
a_id score
1 80
2 70
1 30
左连接：
select A.* ,B.* from A a left outer join B b on a.id = b.a_id;
result:
id name a_id score
1 bai 1 80
1 bai 1 30
2 zhang 2 70
3 li null null
select A.* ,B.* from B b right outer join A a on a.id = b.a_id;
result:
id name a_id score
1 bai 1 80
1 bai 1 30
2 zhang 2 70
3 li null null
select id name from A a union select b.a_id id,score name B b; //删除重复相
select id name from A a union all select b.a_id id,score name B b; //不删除重复项