测试派 Give testers a home.

1：

2：

3：第三题觉得题意有点模糊，我理解的是求职位为CLERK的员工姓名、以及员工所在的部门名称及部门名称中有CLERK员工的人数，不知道理解得对不对，还是这个部门人数只是单纯的部门有的员工数量

1：

2：

1：
select MAX(score) from sc s inner join course c on s.Cno = c.Cno and c.Cname = '语文'

2：
select AVG(s.score),c.Cname from sc s inner join course c on s.Cno = c.Cno
GROUP BY c.Cname;

3：
SELECT * from student s inner join sc c on s.Sno = c.Sno
inner join course r on r.Cno = c.Cno
inner join teacher t on t.Tno = r.Tno where t.Tname = '孤独求败'

1：select * from emp inner join dept on emp.id=dept.id where emp.name like '张%'

2：
select count() from emp as a inner join emp as b on a.leader = b.id where b.name ='张三丰'
select count() from emp as a inner join emp as b on a.leader = b.id and b.name ='张三丰'
发现第二题用where 和and 的结果是一样的

3：select * from emp where dept_id = 0