推荐课程：MySQL教程。

创建示例数据表：

#学生表
create table stu(sid int primary key,sname varchar(10) not null);
#成绩表
create table score (sid int, score int , cid int);
#科目表
create table subjects (cid int primary key,cname varchar(10));
alter table score add constraint fk_score_sid foreign key(sid) references stu(sid);
alter table score add constraint fk_score_cid foreign key(cid) references subjects(cid);

查询 学生成绩 和科目名称

select * from stu,score where stu.sid = score.sid;

查询学生的成绩 要求70以上的

select s.sid, s.sname, c.score from stu s,score c where s.sid = c.sid and c.score>70;

查询所有学生的成绩

select * from stu s inner join score c on s.sid = c.sid; 
内连接相当于把两张表链接成一张表查询 使用inner join 关键词 条件 使用on 并且Inner 可以省略 
外链接查询 左外连接查询 以左边的表为主 左边表中的数据都会查询出来 有可能会产生无用的数据 
select s.sid, s.sname, c.score from score c left join stu s on s.sid = c.sid; 
右外链接查询 
select s.sid, s.sname, c.score from score c right join stu s on s.sid = c.sid; 
自然链接 :会自动匹配表中相同的字段 没有外键也能自然查询 
select * from stu natural join score;

查询所有学生的分数和考试科目

select s.sid, s.sname, c.score,u.cname from stu s left join score c on s.sid = c.sid join subjects u on c.cid = u.cid; 
三张表查询时2条件 n张表n-1条件

创建示例数据表：

#员工表
CREATE TABLE emp(
 empno INT,
 ename VARCHAR(50),
 job VARCHAR(50),
 mgr INT,
 hiredate DATE,
 sal DECIMAL(7,2),
 comm decimal(7,2),
 deptno INT
);
INSERT INTO emp values(7369,'SMITH','CLERK',7902,'1980-12-17',800,NULL,20);
INSERT INTO emp values(7499,'ALLEN','SALESMAN',7698,'1981-02-20',1600,300,30);
INSERT INTO emp values(7521,'WARD','SALESMAN',7698,'1981-02-22',1250,500,30);
INSERT INTO emp values(7566,'JONES','MANAGER',7839,'1981-04-02',2975,NULL,20);
INSERT INTO emp values(7654,'MARTIN','SALESMAN',7698,'1981-09-28',1250,1400,30);
INSERT INTO emp values(7698,'BLAKE','MANAGER',7839,'1981-05-01',2850,NULL,30);
INSERT INTO emp values(7782,'CLARK','MANAGER',7839,'1981-06-09',2450,NULL,10);
INSERT INTO emp values(7788,'SCOTT','ANALYST',7566,'1987-04-19',3000,NULL,20);
INSERT INTO emp values(7839,'KING','PRESIDENT',NULL,'1981-11-17',5000,NULL,10);
INSERT INTO emp values(7844,'TURNER','SALESMAN',7698,'1981-09-08',1500,0,30);
INSERT INTO emp values(7876,'ADAMS','CLERK',7788,'1987-05-23',1100,NULL,20);
INSERT INTO emp values(7900,'JAMES','CLERK',7698,'1981-12-03',950,NULL,30);
INSERT INTO emp values(7902,'FORD','ANALYST',7566,'1981-12-03',3000,NULL,20);
INSERT INTO emp values(7934,'MILLER','CLERK',7782,'1982-01-23',1300,NULL,10);

-- 部门表：dept
CREATE TABLE dept(
 deptno INT,
 dname varchar(14),
 loc varchar(13)
);
INSERT INTO dept values(10, 'ACCOUNTING', 'NEW YORK');
INSERT INTO dept values(20, 'RESEARCH', 'DALLAS');
INSERT INTO dept values(30, 'SALES', 'CHICAGO');
INSERT INTO dept values(40, 'OPERATIONS', 'BOSTON');

查询与 SCOTT 同一个部门的员工

select ename from emp where deptno = (select deptno from emp where ename=’SCOTT’);

工资高于30号部门所有人的员工信息

select * from emp where sal>(select max(sal) from emp where deptno = 30);

查询工作和工资与MARTIN 完全相同的员工信息

select * from emp where (job,sal) in (select job,sal from emp where ename=’MARTIN’);

有两个以上有直接下属的员工信息

select * from emp where empno in (select mgr from emp group by mgr having count(mgr)>=2);

查询员工编号为7788的员工名称 员工工资 部门名称 部门地址

select e.ename,e.sal,d.dname,d.loc from emp e ,dept d where e.deptno = d.deptno and empno = 7788;

求7369员工编号、姓名、经理编号和经理姓名

select ename,empno from emp where empno in (7369, (select mgr from emp where empno = 7369)); 
select e1.ename,e1.empno,e2.ename,e2.empno from emp e1,emp e2 where e1.mgr = e2.empno and e1.empno = 7369

求各个部门薪水最高的员工所有信息

select * from emp where (deptno ,sal) in (select deptno,max(sal) from emp group by deptno); 
查询过程中出现问题 查询时没有把部门的条件带进去 只是按最高工资处理的 可以把查询结果返回的结果集当做一个新表来使用 
select * from emp e1 ,(select deptno,max(sal) msal from emp group by deptno) e2 where e1.sal = e2.msal and e1.deptno = e2.deptno;