myonepractice

select * from EMP where ENAME =‘SMITH‘;
select ename || ‘ is a ‘ || job from emp;
select distinct empno from emp;
select * from emp where COMM>SAL;
select COMM+SAL from emp;
select * from emp where job not in(‘SALESMAN‘,‘MANAGER‘);
select * from emp where COMM is not null;
select ename,sal from emp order by sal; /*默认升序（asc）*/
select upper (‘avdd‘) from dual;
select initcap(ename) from emp;/*将所有单词首字母变大写*/
select concat(‘s‘,‘v‘)from dual;
select concat(‘a‘,‘‘) from dual;
select concat(‘‘,‘a‘) from dual;/*对空格连接不起作用*/
select length(ename) from emp;/*测量某列长度*/
select replace(‘ename‘,‘a‘,‘K‘) from dual;
select instr(‘hello world‘，‘e‘)from dual;/*指定字符出现的位置*/
select concat(JOB,SAL) from emp;
select JOB||SAL from emp;
select * from emp where lower(ENAME)=‘smith‘;
select * from emp where ENAME is null;
select length(ename) from emp;/*列的真实字段名儿*/
/*substr(字符串,截取开始位置,截取长度) //返回截取的字*/
select trunc(49.736,-1) from dual;
select sysdate from dual;
select months_between(to_date(‘10-07-1994‘,‘dd-mm-yyyy‘),to_date(‘11-06-1994‘,‘dd-mm-yyyy‘)) from dual;
select add_months(to_date(‘08-03-2017‘,‘dd-mm-yyyy‘),6) from dual;
select ‘[‘||ENAME||‘]‘ from emp;
select ‘[‘||ENAME||‘]‘ from emp where ename=‘SMITH‘;
select NEXT_DAY(to_date(‘01-09-2017‘,‘dd-mm-yyyy‘),‘星期一‘) from dual;
select last_day(to_date(‘18-08-2017‘,‘dd-mm-yyyy‘)) from dual;
select to_char(sysdate,‘mm‘) from dual;
select to_number(‘13‘)+to_number(‘15‘)from dual;--转换为数字
select round(to_date(‘08-09-2013‘,‘dd-mm-yyyy‘)) from dual;
select round(sysdate) from dual;
--分组函数
select avg(sal) from emp;
select sum(sal) from emp;
select max(sal) from emp;
select min(sal) from emp;
--4.6
select *from emp where last_day(HIREDATE)-2=HIREDATE;
select * from emp where hiredate<=add_months(hiredate,-25*12);--25年前雇的员工
select round(months_between(sysdate,HIREDATE))*30 from emp;
--通用函数
select nvl(COMM,0) from emp;--相同则返回空值，不相同则返回第一个表达式的值
select nvl2(sal,comm+sal,sal) from emp;--第一个值不为空，则返回第二个值；为空则返回第三个值
select sal,coalesce(sal,COMM+SAL,sal,0) from emp;--返回第一个非空值
select empno case deptno 
                  when 10 then ‘财务部‘
                  when 20 then ‘研发部‘
                  else ‘销售部‘
                  end 
                  from emp;
--分组函数
select ename from emp group by DEPTNO,ename;
select avg(sal),deptno from emp group by deptno;
select count(*) from emp;
select avg(sal) from emp group by sal order by sal desc;--order by出现在group by之前没有意义
select * from emp;
select sal,deptno from emp group by sal,deptno;
--多表查询
select * from STUDENT,SC;
select * from(select * from sc,student where student.s#=sc.s#) as a group by a.c#;--false
select student.s#,student.* from student left outer join sc on sc.s#=student.s#;
select /*s#,sname,*/avg(select score from
                               (select student.s#,student.sname,sc.score from student,sc where student.s#=sc.s#(+))xinbiao)
                               from xinbiao
                               group by xinbiao.score;--感觉快要成功了，加油啊！！！
select count(*) from(select student.s#,student.sage,sc.score from student,sc where student.s#=sc.s#(+)); 
select student.s#,student.sname,avg(sc.score)from student,sc where student.s#(+)=sc.s# group by sc.score;
--practice
select emp.deptno,count(*)from emp group by emp.deptno;
select avg(sal)/*,emp.deptno*/from emp where sal>500 group by emp.deptno /*having sal>500*/
--查询显示工资大于各个部门工资的平均值的员工信息
select * from emp e1,(select avg(sal) avgsal from emp)e2 where e1.sal>e2.avgsal; 
select avg(sal),deptno from emp group by deptno having sal>avg(sal);
select count(*),emp.deptno from emp group by emp.deptno;
--查询出每个部门的编号、名称、位置、部门人数、平均工资
select emp.deptno,emp.ename,emp.mgr from emp;
select avg(sal) from emp group by emp.deptno;
--要求查询出工资比SMITH工资要高的全部雇员信息
select * from emp where sal>(select sal from emp where emp.ename=‘SMITH‘);
select * from emp where sal>800;
select* from emp where sal between 0 and 1000;
select emp.sal from emp order by sal desc;
select sal*12 as a from emp;  
select * from emp where job not in(‘CLERK‘);
select * from emp where job not LIKE(‘%CLERK%‘);
select * from emp where emp.ename like(‘%O%T%‘);
select distinct emp.job from emp;
--查询平均成绩大于等于70分的同学的学生编号和学生姓名和平均成绩
select student.s#,student.sname from student,(select avg(sc.score)avgsr from sc group by sc.c#)avgsc where avgsc.avgsr>=70;