RTRIM - Strings 
LTRIM 
Courses : 
Course_name varchar(10)   ITEC212  ----------

        ITEC212---     
select rtrim(course_name)
from courses 

	---ITEC212
select ltrim(course_name)
from courses 

Example: in this example we tried to compare the length of the string 
before and after using RTRIM

select department_name, length(department_name) "Before RTRIM", length(RTRIM(department_name)) "After RTRIM"
from departments

Number Functions: 
Round(number,position) - 7.5 = 8 , 7.3 = 7, 7.65 = 8
Trunc(number,position) - 7.5 = 7.5 , 7.3 = 7, 7.65 = 7.65
Mod(111,6) - 8%2 = 0  ,   5%2 = 1


Example: 
45.635489 - The decimal point represents position zero
Round(45.635489,5) = 45.63549
Round(45.635489,0) = 46
Trunc(45.635489,5) = 45.63548
Trunc(45.635489,2) = 45.6
Round(45.635489,-1) = 46
Trunc(45.635489,-1) = 45
Trunc(44.635489,-1) = 40

Dual table:
you can use this table when you're dealing with numbers 
that are not stroed in the databse, also you can use it 
to display the system information. 

Sysdate : will return the system date 

select sysdate  //will display the system date 
from dual 

Operations with dates:
select sysdate + 24 //will add 24 days to the system date 
from dual 

select sysdate - 5 //will display 29th from last month 
from dual 

Subtract two dates to find out the number of days between
these dates 

Example: 
select first_name, hire_date "Started", round(sysdate - hire_date) "Duration"
from employees 


// use dual table ONLY if you displaying system information 
ALONE without any information from other tables 

Example: Find out for how many years the employees have been 
hired in the company 

SELECT LAST_NAME, ROUND((SYSDATE-HIRE_DATE)/365)
FROM EMPLOYEES

Example 2:
SELECT FIRST_NAME, HIRE_DATE, ROUND(SYSDATE-HIRE_DATE) "NUMBER OF DAYS",
ROUND((SYSDATE-HIRE_DATE)/365) "NUMBER OF YEARS", ROUND((SYSDATE-HIRE_DATE)/12) "NUMBER OF MONTHS",
ROUND((SYSDATE-HIRE_DATE)/7) "NUMBER OF WEEKS"
FROM EMPLOYEES 

Date Functions:
- sysdate
- current_date

select sysdate, current_date //somehow the same 
from dual

- current_timestamp
select CURRENT_TIMESTAMP
from dual