内连接
内连接实际上就是利用where子句对两种表形成的笛卡儿积进行筛选
语法：
select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件；
--where
mysql> select ename,dname from emp,dept where emp.deptno=dept.deptno and ename='SMITH';
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
--内连接
mysql> select ename,dname from emp inner join dept on emp.deptno=dept.deptno and ename='SMITH';
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
外连接
左外连接
如果联合查询，左侧的表完全显示我们就说是左外连接
mysql> select * from stu;
+------+------+
| id | name |
+------+------+
| 1 | jack |
| 2 | tom |
| 3 | kity |
| 4 | nono |
+------+------+
mysql> select * from exam;
+------+-------+
| id | grade |
+------+-------+
| 1 | 56 |
| 2 | 76 |
| 11 | 8 |
+------+-------+
--内连接：拼接符合条件的表
mysql> select * from stu inner join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | jack | 1 | 56 |
| 2 | tom | 2 | 76 |
+------+------+------+-------+
--左外连接：在完整显示左侧的表基础上，显示符合条件的
mysql> select * from stu left join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | jack | 1 | 56 |
| 2 | tom | 2 | 76 |
| 3 | kity | NULL | NULL |
| 4 | nono | NULL | NULL |
+------+------+------+-------+
右外连接
如果联合查询，右侧的表完全显示我们就说是右外连接。
左外连接和右外连接可以转化，注意连接位置就行
--右外
mysql> select * from stu right join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | jack | 1 | 56 |
| 2 | tom | 2 | 76 |
| NULL | NULL | 11 | 8 |
+------+------+------+-------+
--左外
mysql> select * from exam left join stu on stu.id=exam.id;
+------+-------+------+------+
| id | grade | id | name |
+------+-------+------+------+
| 1 | 56 | 1 | jack |
| 2 | 76 | 2 | tom |
| 11 | 8 | NULL | NULL |
mysql> select d.dname,e.* from dept d left join emp e on d.deptno=e.deptno order by d.deptno asc;
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| dname | empno | ename | job | mgr | hiredate | sal | comm | deptno |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
小练习
--where
select dp.emp_no ,dm.emp_no from
dept_emp dp , dept_manager dm
where dp.dept_no=dm.dept_no
and dp.emp_no!=dm.emp_no;
--内连接
select dp.emp_no ,dm.emp_no from
dept_emp dp inner join dept_manager dm on dp.dept_no=dm.dept_no
where dp.emp_no!=dm.emp_no;
分数排名
第一步：查找排名，即去重后比该分数多的行数
第二步：对分数进行降序
select score,
(
--找出排名
select count(distinct score) from Scores where score>=s.score
) as 'rank'
from Scores s
order by score desc;
使用rank（函数）rank()函数及其用法
select score, dense_rank() over (order by score desc) as 'rank' #这个rank之所以要加引号，因为rank本身是个函数，直接写rank会报错
from scores;
换座位 Exchange Seats
小美是一所中学的信息科技老师，她有一张 seat 座位表，平时用来储存学生名字和与他们相对应的座位 id。
其中纵列的 id 是连续递增的
小美想改变相邻俩学生的座位。
你能不能帮她写一个 SQL query 来输出小美想要的结果呢？
+---------+---------+
| id | student |
+---------+---------+
| 1 | Abbot |
| 2 | Doris |
| 3 | Emerson |
| 4 | Green |
| 5 | Jeames |
+---------+---------+
假如数据输入的是上表，则输出结果如下：
+---------+---------+
| id | student |
+---------+---------+
| 1 | Doris |
| 2 | Abbot |
| 3 | Green |
| 4 | Emerson |
| 5 | Jeames |
+---------+---------+
注意：如果学生人数是奇数，则不需要改变最后一个同学的座位。
select
IF(mod(id,2)=0,id-1,IF(id=(select max(id) from seat),id,id+1)) as id,student
from seat
order by id
IF函数：该函数的执行机制是判断第一个参数是否为真，若为真则返回第二个参数，若为假则返回地三个参数
IF（true,0,1），返回0
IF（false,0,1），返回1
本题类似C/C++遍历数组，循环判断id大小，然后改变id