union, union all
-- 第一张表与第二张表纵向地合并，有去重的功能
MariaDB [hellodb]> select * from teachers
-> union
-> select stuid,name,age,gender from students;
MariaDB [hellodb]> select * from teachers union select * from teachers;
-- 纵向合并不去重
MariaDB [hellodb]> select * from teachers union all select * from teachers;
cross join
-- 交叉连接(横向笛卡尔)
MariaDB [hellodb]> select * from students cross join select * from teachers;
inner join
MariaDB [hellodb]> select * from students inner join teachers on students.teacherid=teachers.tid;
MariaDB [hellodb]> select s.stuid,s.name,s.age,t.tid,t.name,t.age from students s inner join teachers t on s.teacherid=t.tid;
MariaDB [hellodb]> select s.stuid,s.name,s.age,t.tid,t.name,t.age from students s,teachers t where s.teacherid=t.tid;
MariaDB [hellodb]> select s.stuid,s.name student_name,s.age student_age,t.tid,t.name teacher_name,t.age teacher_age from students s inner join teachers t on s.teacherid=t.tid and s.age > 30;
left [outer] join
-- 左边表全要，右边的表取left_name 与 right_name的交集 (left outer join)
MariaDB [hellodb]> select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s left join teachers as t on s.teacherid=t.tid;
MariaDB [hellodb]> select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s left outer join teachers as t on s.teacherid=t.tid;
right [outer] join
右边的表全要，左边的表取left_name与right_name的交集 (right outer join)
MariaDB [hellodb]> select s.stuid,s.name,s.age,s.teacherid,t.tid,t.name,t.age from students s right outer join teachers t on s.teacherid=t.tid;
A-(A与B的交集)
MariaDB [hellodb]> select s.stuid,s.name,s.age,s.teacherid,t.tid,t.name,t.age from students as s left outer join teachers as t on s.teacherid=t.tid where t.tid is null;
B-(A与B的交集)
MariaDB [hellodb]> select s.stuid,s.name,s.age,s.teacherid,t.tid,t.name,t.age from students s right outer join teachers t on s.teacherid=t.tid where s.teacherid is null;
A 并 B(full outer join)
MariaDB [hellodb]> select s.stuid,s.name,s.age,s.teacherid,t.tid,t.name,t.age from students s left outer join teachers t on s.teacherid=t.tid
-> union
-> select s.stuid,s.name,s.age,s.teacherid,t.tid,t.name,t.age from students s right outer join teachers t on s.teacherid=t.tid;
(A并B)-(A交B)
MariaDB [hellodb]> select * from (select s.stuid,s.name s_name,s.teacherid,t.tid,t.name t_name from students s left outer join teachers t on s.teacherid=t.tid union select s.stuid,s.name s_name,s.teacherid,t.tid,t.name t_name from students s right outer join teachers t on s.teacherid=t.tid) as a where a.teacherid is null or a.tid is null;
自连接
MariaDB [hellodb]> select * from emp;
+------+----------+--------+
| id | name | leader |
+------+----------+--------+
| 1 | zhangsan | NULL |
| 2 | lisi | 1 |
| 3 | wangwu | 2 |
| 4 | zhaoliu | 3 |
+------+----------+--------+
MariaDB [hellodb]> select e.name,l.name from emp e left outer join emp l on e.leader=l.id;
+----------+----------+
| name | name |
+----------+----------+
| lisi | zhangsan |
| wangwu | lisi |
| zhaoliu | wangwu |
| zhangsan | NULL |
+----------+----------+
三张表内连接
MariaDB [hellodb]> select st.stuid,st.name,sc.courseid,co.course,sc.score from students st inner join scores sc on st.stuid=sc.stuid inner join courses co on co.courseid=sc.courseid;
+-------+-------------+----------+----------------+-------+
| stuid | name | courseid | course | score |
+-------+-------------+----------+----------------+-------+
| 1 | Shi Zhongyu | 2 | Kuihua Baodian | 77 |
| 1 | Shi Zhongyu | 6 | Weituo Zhang | 93 |
| 2 | Shi Potian | 2 | Kuihua Baodian | 47 |
| 2 | Shi Potian | 5 | Daiyu Zanghua | 97 |
| 3 | Xie Yanke | 2 | Kuihua Baodian | 88 |
| 3 | Xie Yanke | 6 | Weituo Zhang | 75 |
| 4 | Ding Dian | 5 | Daiyu Zanghua | 71 |
| 4 | Ding Dian | 2 | Kuihua Baodian | 89 |
| 5 | Yu Yutong | 1 | Hamo Gong | 39 |
| 5 | Yu Yutong | 7 | Dagou Bangfa | 63 |
| 6 | Shi Qing | 1 | Hamo Gong | 96 |
| 7 | Xi Ren | 1 | Hamo Gong | 86 |
| 7 | Xi Ren | 7 | Dagou Bangfa | 83 |
| 8 | Lin Daiyu | 4 | Taiji Quan | 57 |
| 8 | Lin Daiyu | 3 | Jinshe Jianfa | 93 |
+-------+-------------+----------+----------------+-------+
子查询
-- 大于平均年龄的学生信息
MariaDB [hellodb]> select * from students where age > (select avg(age) from students);
-- 将老师表的平均年龄的值赋给25号学生
MariaDB [hellodb]> update students set age=(select avg(age) from teachers) where stuid=25;
来源：CSDN
作者：studywinwin
链接：https://blog.csdn.net/studywinwin/article/details/104076876