Oracle 查询及高级查询

一、简单查询：

select * from userinfo;
select username,salary from userinfo;
select username,salary+200 from userinfo;
select username from userinfo where salary>800 and salary <>1500.5;
select * from userinfo where username='aaa';
select * from userinfo where not(username='aaa');
select * from userinfo where username like '%a%';
select * from userinfo where username like '_a%';
select * from userinfo where salary between 800 and 2000;
select * from userinfo where salary not(between 800 and 2000);
select * from userinfo where salary in(800,2000,3000);
select * from userinfo where salary not in(800,2000,3000);
select * from userinfo order by id desc/asc;
select * from userinfo order by id desc,username asc;

select username,case username 
when 'aaa' then '计算机部门'
when 'bbb' then '市场部'
else '其他部门' 
end as 部门
from userinfo;

或者：

select username,case 
when username= 'aaa' then '计算机部门'
when username=  'bbb' then '市场部'
else '其他部门' 
end as 部门
from userinfo;

设置别名：
select id as 编号,username as 用户名,salary as 工资 from userinfo; -设置字段别名
select u.id,u.username,u.salary from userinfo u; -设置表别名

二、高级查询：
1、分组查询：(分组函数会自动过滤空值)
1）常用的分组函数：AVG,SUM,MIN,MAX,COUNT,WM_CONCAT
SELECT AVG(SALARY),SUM(SALARY) FROM EMPLOYEE; -求表中工资（字段）的平均值和和
select max(salary),min(salary) from employee; --求最大最小值
select count(*) from employee; --查询表中总记录数
select count(distinct deptno) from employee; -查询不重复部门号的个数
select sum(sal)/count(*) as 一,sum(sal)/count(sal) as 二,avg(sal) as 三 from emp;
select avg(nvl(sal,0)) from emp; --nvl函数表示如果sal为空，则返回0，否则返回sal
2）group by语句
select deptno,avg(salary) from emp group by depno; -求各个部门的平均工资
select sum(salary) from emp group by deptno,job; -求各个部门、各个职位的工资总和
select avg(salary) from emp group by deptno,job having avg(salary)>2000; -having过滤结果，where子句中不能使用分组函数
select avg(salary) from emp group by deptno,job order by avg(salary); -按照部门、职位查询平均工资，并按照平均工资排序
3）group by语句的增强：语法 group by (a,b); 主要用在报表功能中
select deptno,job,sum(salary) from emp group by rollup(deptno,job);
2、多表查询：（笛卡尔集的应用！！！）
select e.eid,e.username.e.salary,d.deptname from emp e,dept d where e.deptno=d.deptno; -等值连接（连接条件）
外连接：（核心：通过外连接，可以把对于连接条件不成立的记录，仍然包含在最后的结果中）
左外连接：（当连接条件不成立的时候，等号左边的表仍然被包含在记录当中）写法：在等号的右边写 (+) 例如:where e.deptno=d.deptno(+);
右外连接：（当连接条件不成立的时候，等号右边的表仍然被包含在记录当中）写法：在等号的左边写 (+) 例如:where e.deptno(+)=d.deptno;
select d.name,d.job from emp e,dept d where e.deptno(+)=d.deptno;
自连接：（核心：通过别名，将同一张表视为多张表）（注意：自连接不适合操作大表）
select e.ename 员工姓名,b.ename 老板姓名 from emp e,emp b where e.mgr=b.empno;
3、子查询：(select语句的嵌套)
select salary from emp where salary > (select salary from emp where ename = 'scott');
注：子查询可以放在select、where、having、from后面，但不能放在group by后面
select * from (select eno,ename,salary,salary*12 年薪 from emp);
1）单行子查询(使用单行操作符：=，>,<等)：返回一条记录的子查询
select * from empwhere job = (select job from emp where empno = 7566) and salary > (select salary from emp where empno = 7782);
2）多行子查询（使用多行操作符：in，any，all）：返回多条记录的子查询
select * from emp where deptno in (select deptno from dept where dname = 'sales' or dname = 'accounting');
select * from emp where salary > any (select salary from emp where dno = 30);
3）多行子查询的空值问题：
select * from emp where eno not in (select mgr from emp); -子查询中有空值，所以返回不了记录
select * from emp where eno not in (select mgr from emp where mgr is not null); -去掉子查询里面的空值，就可以了

三、综合案例：
1、分页查询显示员工信息：员工号、姓名、月薪
-每页显示四条记录
-显示第二页的员工
-按照月薪排序

select r,eno,ename,salary
from(
    select rownum r,eno,ename,salary
    from (select rownum,eno,ename,salary
              from emp 
              order by salary desc) e1
              where rownum <= 8) e2
where r>=5

Oracle 查询及高级查询

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