一．简单SQL查询：
1）:统计每个部门员工的数目
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select dept,count(*) from employee group by dept;
select dept,count(*) from employee group by dept;
2）:统计每个部门员工的数目大于一个的记录
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select dept,count(*) from employee group by dept having count(*)>1;
select dept,count(*) from employee group by dept having count(*)>1;
3）:统计工资超过1200的员工所在部门的名称
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select e.first_name,salary,d.name
from s_emp e, s_dept d
where e.dept_id = d.id
and salary > 1200;
select e.first_name,salary,d.name
from s_emp e, s_dept d
where e.dept_id = d.id
and salary > 1200;
4）:查询哪个部门没有员工
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select e.empno, d.deptno
from emp e, dept d
where e.deptno(+) = d.deptno
and e.deptno is null;
select e.empno, d.deptno
from emp e, dept d
where e.deptno(+) = d.deptno
and e.deptno is null;
二．复杂SQL查询
有3个表（15分钟）：(SQL)
Student 学生表 (学号，姓名，性别，年龄，组织部门)
Course 课程表 (编号，课程名称)
Sc 选课表 (学号，课程编号，成绩)
表结构如下：
1) 写一个SQL语句，查询选修了’JAVA’的学生学号和姓名（3分钟）
答：SQL语句如下：
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select stu.sno, stu.sname
from student stu, course c, sc
where stu.sno = sc.sno
and sc.cno = c.cno
and c.cname=”JAVA”;
select stu.sno, stu.sname
from student stu, course c, sc
where stu.sno = sc.sno
and sc.cno = c.cno
and c.cname=”JAVA”;
2) 写一个SQL语句，查询’a’同学选修了的课程名字（3分钟）
答：SQL语句如下：
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select stu.sname, c.cname
from student stu, course c, sc
where stu.sno = sc.sno
and sc.cno = c.cno
and stu.sname = ”a”;
select stu.sname, c.cname
from student stu, course c, sc
where stu.sno = sc.sno
and sc.cno = c.cno
and stu.sname = ”a”;
3) 写一个SQL语句，查询选修了5门课程的学生学号和姓名（9分钟）
答：SQL语句如下：
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select stu.sno, stu.sname
from student stu
where (select count(*) from sc where sno=stu.sno) = 5;
select stu.sno, stu.sname
from student stu
where (select count(*) from sc where sno=stu.sno) = 5;
三. 在SQL中删除重复记录的方法:（用到rowid (oracle伪列)）1）通过建立临时表来实现
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SQL>create table temp_emp as (select distinct * from employee)
SQL>truncate table employee; (清空employee表的数据）
SQL>rename temp_emp to employee; (再将表重命名）
SQL>create table temp_emp as (select distinct * from employee)
SQL>truncate table employee; (清空employee表的数据）
SQL>rename temp_emp to employee; (再将表重命名）
2）通过使用rowid来实现。
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SQL>delete from employee where rowid not in (
select max(t1.rowid) from employee t1 group by
t1.emp_id,t1.emp_name,t1.salary);–这里用min(rowid)也可以。
SQL>delete from employee where rowid not in (
select max(t1.rowid) from employee t1 group by
t1.emp_id,t1.emp_name,t1.salary);–这里用min(rowid)也可以。
四. TOP N问题：（用到rownum (oracle伪列)）
–rownum只能使用<=或<的关系比较运算符
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select * from s_emp where rownum <= 2;
select * from s_emp where rownum <= 2;
–查询公司工资最高的3个人
/*select * from emp
where rownum <= 3
order by sal desc;*/ 错误的
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select * from (select * from emp order by sal desc)
where rownum <= 3;
select * from (select * from emp order by sal desc)
where rownum <= 3;
五.分页查询：–查询第1-5条记录
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select * from (select rownum num, s_emp.* from s_emp)
where num >=1 and num <= 5;
select * from (select rownum num, s_emp.* from s_emp)
where num >=1 and num <= 5;
–按工资排序，五条一页,查找第二页
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select salary,first_name
from(
select s.*, rownum rm
from (select *
from s_emp
order by salary d
) s
)
where rm between 6 and 10;