数据库的常见查询操作

数据库都通用这是orcl的

–员工表信息

select *from emp;

–部门表的所有信息

select *from dept;

–查询员工表的所有编号，名字，工资

select empno,ename,sal from emp;

–查询员工表的编号，名字，工资，年薪

select empno,ename,sal,sal*12 from emp;

–as别名小名

select empno as 员工编号，ename as 员工姓名，sal as 员工工资 from emp;

–双引号的作用

select empno “en”from emp;

–连接符||

select ename ||’的月薪是’||sal from emp;

–去除重复的行

select distinct ename,sal from emp;

–order by 进行排序asc 升序desc降序

–查询所有数据对工资进行升序排序

select *from emp order by sal asc;

–降序

select *from emp order by sal desc;

–多字段排序分主次

select *from emp order by sal asc,empno desc;

select ename as 姓名，empno as 编号 ，sal as 工资 from emp sal order by 编号 desc, 工资 asc;

–where条件查询

–查询名叫SCOTT的员工信息

select *from emp where ename =’SCOTT’;

–查询工资为1250

select *from emp where sal=1250;

–查询入职时间1981-2-22的信息

select *from emp where hiredate=’22-2月-81’;

–大于

select *from emp where hiredate >’1-1月-81’;

–小于

select *from emp where hiredate <’1-1月-81’;

–查询工资在1250及以上的员工

select *from emp where sal>=1250;

–查询工资不等于800的员工

select *from emp where sal!=800;

select *from emp where sal<>800;

–查询工资在800-1000；

select *from emp where sal between 800 and 1000;

select *from emp where sal between 800 and 1000;

–in集合

–查询800-900-1000-1250的员工信息

select *from emp where sal in(800,1000,900,1250);

–查询奖金为空的员工信息

select *from emp where comm is null;

select *from emp where comm=”;

–查询奖金不为空的

select *from emp where comm is not null;

select *from emp where comm !=”;

–like模糊查询通配符

–查询以s开头的

select *from emp where ename like ‘S%’;

select *from emp where ename like ‘D__%’;

–用escape来强调是,

select *from emp where ename like ‘%\%’escape’\’;

–and or not 运算符

–查询工资大于900并且以s开头

select *from emp where sal>900 and ename like ‘S%’;

–或者以s

select *from emp where sal>900 or ename like ‘S%’;

–常用字符函数

–查询员工表的姓名并且首字母大写

select initcap(ename)from emp;

–小写

select lower(ename)from emp;

–全大写

select upper(ename)from emp;

–左移除

select ltrim(‘scdn’,’sc’)from dual;

–右边移除

select rtrim(‘csdn’,’dn’)from dual;

–左右移除空格

select ltrim(rtrim(’ bfdhugf ‘))from dual;

–翻译

select translate(‘dfsff’,’df’,’lj’)from dual;

–替换

select replace(‘dfsfd’,’df’,’dss’)from dual;

–查找出现的位置

select instr(‘fdsafs’,’s’)from dual;

–截取

select substr(‘sgfyusg’,3,2)from dual;

–连接

select concat(‘dfdsfd’,’fdsfd’)from dual;

select ‘dfdf’||’dfdfa’from dual;

–绝对值

select abs(-15)from dual;

–x的y次幂

select power(2,3)from dual;

–向上取整

select ceil(12.5)from dual;

–向下取整

select floor(12.5)from dual;

–截断保留

select trunc(12.456,2)from dual;

–四舍五入

select round(122.456,2)from dual;

–开平方

select sqrt(4)from dual;

–取余数

select mod(10,3)from dual;

–取符号

select sign(-25)from dual;

select sign(12)from dual;

–两个日期间隔月份

select months_between(sysdate,’8-12月-1998’)from dual;

–修改月份

select add_months(sysdate,1)from dual;

select add_months(sysdate,-1)from dual;

–返回指定的一个星期几的日期

select next_day(‘16-8月-17’,’星期日’)from dual;

–日期的四舍五入

select round (sysdate,’year’)from dual;

select round (sysdate,’month’)from dual;

select round (sysdate ,’day’)from dual;

–日期转字符串

select to_char(sysdate,’yyyy-mm-dd’)from dual;

–字符串转日期

select to_date(‘2017年8月16日’,’yyyy”年”mm”月”dd”日”’)from dual;

–字符串转数值型

select to_number(‘123,549,654,12.789 ′ , ′ 999,999,999,99.999’)from dual;

select to_number(‘￥123,456,46.4567’,’L999,999,99.9999’)from dual;

–数值转字符串

select to_char(12345678.123,’L999,999,99.999’)from dual;

–时间比较问题

select *from emp where hiredate>’1-1月-81’;

select *from emp where hiredate>to_date(‘1981-1-1’,’yyyy-mm-dd’);

select *from emp where to_char(hiredate,’yyyy-mm-dd’)>
4000
’1998-1-1’;

–空转数

select ename,nvl(comm,0)from emp;

–nvl2(e，n,m)e为空m不为空转n

select ename,nvl2(comm,comm,0)from emp;

select decode(sal,1250,’有钱’,5000,’真有钱’,’穷鬼’)from emp;

–case when else end (if else)

select (case

when sal >=5000 then ‘有钱’

when sal>=4000 then ‘1有钱’

when sal>=3000 then ‘2有钱’

else ‘穷’

end)from emp;

select (case sex when 0 then’男’when 1 ‘女’ end)from dual;

–sum求和 max最大值min最小值avg平均值count统计

select max(sal)from emp;

select min(sal)from emp;

select avg(sal)from emp;

select sum(sal)from emp;

select count(sal)from emp;

–多少员工

select count(*)from emp;

–聚合函数可以写在一行上

select max(sal),min(sal),sum(sal),avg(sal),count(*)from emp;

–统计所有（*）

select count(*)from emp;

select count (distinct sal)from emp;

–查寻部门编号为10的员工信息

select *from emp;

select *from emp where deptno like ‘10%’;

–查寻年薪大于三万姓名雨部门编号

select ename,empno,sal*12 from emp where sal*12>30000;

–查询佣金为null姓名工资

select ename as 姓名,sal as 工资 from emp where comm is null;

–查询工资大于1500and有佣金的人

select ename as 姓名, sal as 工资 from emp where sal>1500 and comm is not null;

–查询工资大于1500or有佣金的人

select ename as 姓名, sal as 工资 from emp where sal>1500 or comm is null;

–查询姓名里有S的员工的信息工资名字

select ename as 姓名, sal as 工资 from emp where ename like ‘S%’;

–查询以j开头第二个字符是o 的

select ename as 姓名, sal as 工资 from emp where ename like ‘jo%_’;

select*from emp where ename like ‘% ’;