DBA操作

sqlplus sys/tiger  as sysdba;

alter user scott account unlock;

用户已更改

切换用户：conn scott/tiger as sysdba;

修改密码：alter user scott indentified by tiger;

1.select语句：

select sysdate from dual;

select ename,sal*12+comm from emp;(结果是错的，如果comm为空，那么sal*12+comm整个为空)

select ename ,sal*12 year_sal from emp;

select ename||sal from emp;

select ename || 'dhfaf' from emp;

select ename || 'jfkjdk''kdjfkla' from emp;

distinct:

select distinct deptno from emp; //重复的编号去除掉

select distinct deptno,job from emp; //查询的结果会出现重复的编号和job，但deptno+job不重复

2.where:

select ename,sal,comm from emp where comm is null;

select ename,sal,comm from emp where comm is not null;

select ename,sal,comm from emp where sal in (800,1500,2000);

select ename,sal,comm from emp where sal in ('Simth','abc','ABC');

如果想查询大于2007年2月25日以后入职的员工信息

首先select sysdate from emp;

查询结果是25-2月-07

必须得按这个字符串特定的格式来写：

select ename,sal,hiredate from emp where hiredate >'25-2月-07';

select ename,sal,from emp where deptno =10 and/or sal>1000;

where ename,sal from emp where sal not in (800,1500); //取反

select ename from emp where ename like '%ALL%'; //%表示0个或多个字母

select enaem from emp where ename like '_A%'; //_表示占位符

如果名字里面本身就有%：

select ename from emp where ename like '%$%%' escape '$'; //指定$为转义字符

3.order by

select * from dept order by deptno desc; //降序排列

select empno,ename from emp order by empno asc; //升序排列

select empno,ename from emp where empno <>10 order by empno asc;//先过滤再排序

按照两个字段排序：

select ename,sal,deptno from emp order by deptno asc,ename desc; //先按deptno升序排列再在相同deptno中ename用降序排列

4.函数lower

select lower(ename) from emp;

查询员工名字不管大小写，第二个字母是a的名字：

select ename from emp where lower(ename) like '_a%';

或者：select ename from emp where ename like '_a%' or ename like '_A%';

5.函数substr 截取

select substr(ename,1,3) from emp; //从第一个字符开始截取，长度为3

6.函数chr 把而刺客码转换成字符

select chr(65) from dual; //查询结果为A

7.函数ascii 把字符转换成ASCII

select ascii('A') from dual; //查询结果为65

8.函数round 四舍五入

select round(23.652) from dual; //24

select round(23.652,2) from dual; //23.65

select round(23.652,1) from dual; //23.7

select round(23.652,-1) from dual; //20

*9.函数to_char 把数字/日期转换成相关字符串并且有特定的格式控制

select to_char(sal,'$99,999.9999') from emp;

select to_char(sal,'L99,999.9999') from emp; //人民币

select to_char(hiredate,'YYYY-MM-DD HH24:MI:SS') from emp;

*10.to_date 把特定格式的字符串转换成日期类型

select ename,hiredate from emp where hiredate >to_date('1981-2-20 12:34:56','YYYY-MM-DD HH24:MI:SS');

*11.to_number 把特定格式的数字转换成相关类型

select sal from emp where sal > to_number('$1.250.00','$9,999.99');

*12.nvl 处理空值的

select ename,sal*12 + nvl(comm,0) from emp;

*13.组函数：输入好多条记录，输出只有一条

max(sal) min(sal) avg(sal) sum(sal)

select to_char(avg(sal),'99999.99999') from emp;

select round(avg(sal),2) from emp;

count(*) //表示表中有多少条记录(不是空值的有多少个)

select count(*) from emp where deptno=10;

select count(distinct deptno) from emp; //部门编号有多少个

*14.group by

哪个部门的平均薪水是多少：

select deptno,avg(sal) from emp group by deptno;

按照多个字段进行分组：

select deptno,job,max(sal) from emp group by deptno,job;

输出薪水最高的员工的名字：

错误的是：select ename，max(sal) from emp;

真确的是：select ename from emp where sal=(select max(sal) from emp);

输出每个部门挣钱最多人的名字

错误的是：select ename,max(sal) from emp group by deptno;

真确的是：select deptno,max(sal) from emp group by deptno; //deptno在分组里面确确实实只有一个

*15.having 是对分组进行限制

where语句是对单条的记录进行过滤

把平均薪水大于2000的部门取出来

select avg(sal）,deptno from emp group by deptno having avg(sal)>2000;

顺序：select * from emp

where sal>1000

group by deptno

having

order by

例子：薪水大于1200的雇员按照部门编号进行分组，分组后的平均薪水必须大于1500，查询分组后的平均工资，按工资的倒叙排列

select avg(sal) from emp where sal>1200 group by deptno having avg(sal)>1500 order by avg(sal) desc;

*16.子查询：在一个select语句里面套了个另外的select语句

套的select语句可以出现在where里面，也可以出现在from语句后面

求哪些人的工资位于所有平均工资之上

select ename,sal from emp where sal>(select avg(sal) from emp);

按照部门进行分组后，每个部门里面挣钱最多的员工信息

select ename,sal,deptno from emp

where sal=(select max(sal) from emp group by deptno);//错误的

select ename,sal,deptno from emp

where sal in (select max(sal) from emp group by deptno);//错误的

select ename,sal from emp

join (select max(sal)  max_sal,deptno from emp group by deptno ) t

on (emp.sal=t.max_sal and emp.deptno=t.deptno); //正确的

*17.自连接：其实是在同一张表内进行两次查询.为同一张表其不同别名，当成两张表来用

找出员工对应的经理的名字（分析：员工名字和经理名字都在emp表的ename列中）

select t1.ename,t2.ename from emp t1 ,emp t2 where t1.mgr=t2.empno;

*18.

等值连接：找出每位员工的部门名称

select ename,dname from emp join dept on (emp.deptno=dept.deptno);

求部门平均薪水的等级

select deptno,avg_sal,grade from

(select avg(sal) avg_sal,deptno from emp group by deptno) t

join salgrade s on (t.avg_sal between s.losal and s.hisal);

求部门平均的薪水等级

select deptno ,avg(grade) from

(select deptno,ename,grade from emp join salgrade s on (emp.sal between s.losal and s.hisal)) t

group by deptno;

雇员中有哪些人是经理

select ename from emp where empno in (select distinct mgr from emp);

面试题：

1.不用组函数，求出薪水的最高值（自连接）

左边的一张表中的薪水小于右边一张表的薪水，有一条或多条是连接不上右边，就是左边的最大值没有小于右边表的薪水

select distinct sal from emp where sal not in

(select distinct e1.sal from emp e1 join emp e2 on (e1.sal<e2.sal));

2.平均薪水最高的部门的部门编号

select deptno,avg_sal from

(select avg(sal) avg_sal,deptno from emp group by deptno) t

where avg_sal=

(select max(avg_sal) from t);

3.平均薪水最高的部门的部门

select dname from dept where deptno =

(

select deptno,avg_sal from

(select avg(sal) avg_sal,deptno from emp group by deptno) t

where avg_sal=

(select max(avg_sal) from t)

);