1:常用的函数
to_date()函数，将字符串转换为日期格式
select to_date('2015-09-12','yyyy-MM-dd') from dual; --其中后面的日期格式要和前面要转化的匹配
to_number()函数，将字符串转换为数字格式
select ename,sal from emp where sal>to_number('$5000.00','$9999.99');
清屏命令：clear screen;
2:常用的组函数 max() min() avg() sum() count()
select max(sal) from emp; --最高薪水是多少
select min(sal) from emp; --最低薪水是多少
select avg(sal) from emp; --平均薪水是多少
select sum(sal) from emp; --薪水总和是多少
select count(ename) from emp; --总共有多少员工
3:group by 求部门的平均薪水
select deptno,avg(sal) from emp group by deptno;
4：执行select一般顺序
select deptno,avg(sal) from emp(表名) where sal>5000(where限定条件) group by deptno(按照部门分组) having avg(sal)>5000
order by avg(sal) desc;
5：处理重复记录的问题
select empno from emp group by empno having count(empno)>1; --查询出重复的empno
select * from emp where empno in(select empno from emp group by empno having count(empno)>1); --查询出重复的记录
delete from emp where empno in (select empno from emp group by empno having count(empno)>1) and rowid not in (select min
(rowid) from emp group by empno having count(empno)>1); --删除重复记录，保留第一条
6:子查询
select ename,sal from emp where sal=(select max(sal) from emp); --查询公司中薪水最高的员工姓名
select ename,sal from emp where sal in(select max(sal) from emp group by deptno);--查询每个部门薪水最高的员工姓名 --错误的
（所有员工只要和其中一个匹配就会被查出）
select ename,sal from emp e join (select max(sal) max_sal,deptno from emp group by deptno) t on (e.sal=t.max_sal and
e.deptno=t.deptno);
或者select ename,sal from emp e,(select max(sal) max_sal, deptno from emp group by deptno) t where e.sal=t.max_sal and
e.deptno=t.deptno;
select empno,count(1) from emp group by empno having count(1)>1; --查询员工号重复的员工
92年标准：
select ename,sal,grade from emp e,grade g where e.sal between g.lowsal and g.higsal; --求所有员工薪水等级
select ename,sal,grade from emp e join (select max(sal) max_sal,deptno from emp group by deptno) t on (e.sal=t.max_sal and
e.deptno=t.deptno) join grade g on (t.max_sal bwtween g.lowsal and g.higsal);--求部门中薪水最高员工姓名、等级
--求各部门平均工资等级
select deptno ,avg_sal,grade from (select avg(sal) avg_sal,deptno from emp group by deptno) t join grade g on (t.avg_sal
between g.lowsal and g.higsal);
--求各部门的平均薪水等级
select ename,deptno,sal,grade from emp e join grade g on(e.sal between g.lowsal and g.higsal); --每个员工的薪水等级
select deptno,avg(grade) from (select ename,deptno,sal,grade from emp e join grade g on(e.sal between g.lowsal and
g.higsal)) t group by deptno;
99年sql标准：
select ename,sal,grade from emp e join grade g on (e.sal between g.lowsal and g.higsal);
自连接：在同一张表进行操作
select e1.ename,e2.ename from emp e1 join emp e2 on (e1.mgr=e2.empno);或者select e1.ename,e2.ename from emp e1,emp e2 where
e1.mgr=e2.empno;
左外连接：
select e.ename,d.loc from emp e left join dept d on (e.deptno=d.deptno);--左外连接会把左边那张表中没有关联到的字段也给查询
出来
右外连接：
select e.ename,d.loc from emp e right join dept d on(e.deptno=d.deptno);--右外连接会把右边那张表中没有关联到的字段也给查询
出来
全外连接：
select e.ename,d.loc from emp e full join dept d on(e.deptno=d.deptno); --全连接会把左右两边表中没有关联到的字段都给查询出
来
7:--查询所有的经理人
select ename from emp where empno in(select distinct mgr from emp);
8:--不使用组函数，查询薪水的最高值(先按照薪水排序，然后查询第一条)
select sal from (select sal from emp order by sal desc) where rownum=1;
或者
select e1.sal from emp where sal not in (select e1.sal from emp e1 join emp e2 on (e1.sal<e2.sal)); --自连接表，那么表e1中
只有最大值不会关联到
9：--求平均薪水最高的部门的名称
1：--求各部门平均薪水
select deptno,avg(sal) from emp group by deptno;
2: --求平均薪水最高为多少
select max(avg_sal) from (select deptno,avg(sal) avg_sal from emp group by deptno);
3: --求平均薪水最高部门的编号
select deptno from (select deptno,avg(sal) avg_sal from emp group by deptno) where avg_sal =(select max(avg_sal) from
(select deptno,avg(sal) avg_sal from emp group by deptno));
4:--求平均薪水最高部门的名称
select dname from dept where deptno = (select deptno from (select deptno,avg(sal) avg_sal from emp group by deptno) where
avg_sal =(select max(avg_sal) from (select deptno,avg(sal) avg_sal from emp group by deptno)));
10: --求平均薪水等级最低的部门名称
1:--求各部门平均薪水
select avg(sal),deptno from emp group by deptno;
2:--求各部门平均薪水等级
select deptno,avg_sal,grade from (select avg(sal) avg_sal,deptno from emp group by deptno) t join grade g on(t.avg_sal
between g.lowsal and g.higsal);
3:--求平均薪水等级最低
select min(grade) from (select grade from (select avg(sal) avg_sal,deptno from emp group by deptno) t join grade g on
(t.avg_sal between g.lowsal and g.higsal));
4:--求平均薪水等级最低的部门编号
select deptno from (select deptno,avg_sal,grade from (select avg(sal) avg_sal,deptno from emp group by deptno) t join grade
g on(t.avg_sal between g.lowsal and g.higsal)) where grade =(select min(grade) from (select grade from (select avg(sal)
avg_sal,deptno from emp group by deptno) t join grade g on(t.avg_sal between g.lowsal and g.higsal)));
5:--求平均薪水等级最低的部门编号
select dname from dept where deptno in (select deptno from (select deptno,avg_sal,grade from (select avg(sal)
avg_sal,deptno from emp group by deptno) t join grade g on(t.avg_sal between g.lowsal and g.higsal)) where grade =(select
min(grade) from (select grade from (select avg(sal) avg_sal,deptno from emp group by deptno) t join grade g on(t.avg_sal
between g.lowsal and g.higsal))));
11:oracle数据库用户切换
conn sys/root(用户名/密码) as sysdba;
授权语句：
grant create table,create view to system(用户名);
grant select,update,insert on emp to hr; --一次可以授权多种操作，但是只能针对一张表
12：--求比普通员工最高工资还要高的经理人
1：求普通员工的最高工资
select max(sal) from emp where empno not in(select distinct mgr from emp where mgr is not null)
2：求还要高的经理人
select ename from emp where empno in (select distinct mgr from emp where mgr is not null) and sal > (select max(sal) from
emp where empno not in(select distinct mgr from emp where mgr is not null));
13:--求薪水最高的前2名员工
select ename,sal from (select ename,sal from emp order by sal desc) where rownum <3;
14:rownum 子查询，实现分页，首先是排序，然后是查询rownum，第三步限定条件
select ename,sal from
(select ename,sal,rownum r from
(select ename,sal from emp order by sal desc)) where r>=2 and r<=3
对比：
mysql：select ename,sal from (select ename,sal from emp order by sal desc) t where id limit 2,2;--从2开始取，取2条
db2： 使用fetch
13：dml语句(数据操纵语句)
update emp set ename='tom' where empno=1001;
delete from emp where empno=1002;
insert into emp (empno,deptno) values(1005,20);
14:事务控制（dcl语句）
a：commit提交 rollback回滚 transcation事务结束
b：当执行ddl语句之后，之前的事务也会被提交
c：正常的退出连接，exit，也会自动提交事务