Avg ( ) function is used to return the average of numeric column . If in a table there is null value then it ignored that value .
Syntax :

select avg ( column_name ) from table_name where clause ; // where clause is optional

Example :
Table name : Employee_Info

  Id        Employee_name          Employee_Age               Employee _Salary
  1            Mukesh                  23                           100000
  2            Ayush                   24                           200000
  3            Ishan                   20                           400000
  4            Pranav                  35                           700000
  5            Abhishek                26                           800000
  6            Ravi                    25                           300000
  7            David                   40                           800000

Query to find the avg of employee salary :

select avg ( Employee_Salary ) from Employee_Info ;

Output :

Average of Employee_Salary is  :
471428.571429