select * from EMp--all data in EMP table
select * from EMP where ename in(‘SMITH‘)--the data where ename is SNITH in the EMP table
select ename||‘is a‘ as INAME from EMP where eNAME = ‘SMITH‘--rename ENAME pius ‘is a‘ to INAME
select sal || ‘_‘||ename as employees from emp --sal plus ename to employees
select distinct sal from emp--delete the same rows
select *  from emp order by sal desc--show the all data in the sal‘s data to up order
select * from emp where hiredate = ‘03-12月-81‘--show the data when the hiredate is ‘1981-12-03‘
select *from emp 
       where sal between 1100 and 3000 --select the data of sal 1100 to 3000
select * from emp 
       where deptno in (10,20)--where the deptno is 10 or 20
select * from emp where ename like ‘%_‘--maybe
select * from emp where comm is null--nothing
select * from emp where lower (ename) = ‘smith‘ -- change the word to lowercase
select initcap(ename) from (select lower(ename) as ename from emp)--change the first letter to capital
select concat(‘hello ‘,‘world‘)from dual--attach the two word
select substr(‘goodgoodstudydaydayup‘,5,9)from dual--cut nine letter start for five from the words
select length(‘goodgoodstudydaydayup‘)from dual--how many letter in the words
select instr(‘goodgoodstudydaydayup‘,‘s‘)from dual--the index of the letter ‘s‘ in the words
select lpad(‘goodgoodstudydaydayup‘,30,‘-‘)from dual--make the words has 30 letters with the assignation words in the end,others fill in ‘-‘
select rpad(‘goodgoodstudydaydayup‘,30,‘-‘)from dual--make the words has 30 letters with the assignation words in the beginning,others fill in ‘-‘
select trim(‘p‘ from ‘goodgoodstudydaydayup‘)from dual--delete the letter ‘p‘ from the starting or the end of the words
select round(3.1415926535,4)from dual--retention 4 decimal point with rounding of the number
select trunc(3.1415926535,4)from dual--cut 4 decimal point from the number
select mod(10,3)from dual --get the remainder
select sysdate from dual--get the currentdate of the system
select hiredate+5 from emp--the date plus 5,result is new date
select months_between(‘01-8月-95‘,‘11-7月-94‘)from dual--the defference of the two date 
select add_months(‘01-7月-90‘,7)from dual--the date add to 7 months
select * from emp order by sal desc--descending order
select next_day(sysdate,‘星期一‘)from dual--the date of the next monday
select last_day(sysdate)from dual--the last day of the month of the assignation date
select replace(‘goodgoodstudydaydayup‘,‘d‘,‘y‘)from dual--replace ‘d‘ to ‘y‘
select to_char(sysdate,‘yyyy‘)from dual--the year of currentdate
select to_char(sysdate,‘fmyyy-mm-dd‘)from dual--format the system date to year-month-day
select to_char(sal,‘$999,999,999‘)from emp--format the currency
select to_number(‘13‘)+to_number(‘23‘)from dual--transform varchar to number
select to_date(‘19900725‘,‘yyyymmdd‘)from dual--transform from varchar to date
select *from emp where hiredate = last_day(hiredate)-2--reciprocal 3 days
select *from emp where hiredate <= add_months(sysdate,-25*12)--25years ago
select initcap(concat(‘dear‘,ename)) from emp--the front of all ename add to ‘dear‘,the change the first letter to capital
select * from emp where length(ename) = 5--whose the length of name is 5
select *from emp where ename not like (‘%R%‘)--whose name without the letter ‘R‘
select substr(ename,0,1) from emp --select the first letter of name