AGGREGATE FUNCTION

(i) Aggregate functions perform a calculation on a set of values and return a single value.
(ii) Aggregate functions return the same value any time that they are called by using a specific set of input values.
(iii) Aggregate functions can be used as expressions only in the following:
        (a) The select list of a SELECT (either a subquery or an outer query).
        (b) A HAVING clause.
     
   AVG() Function:

      The AVG() Function returns the average value of a numeric column.
       Syntax: SELECT AVG (column_name) FROM table_name

  COUNT() Function:
       The COUNT() Function returns the number of rows that matches a specified criteria.
        COUNT (column_name) syntax:
        It returns the number of values (NULL values will not be counted) of the specified column:
        SELECT COUNT (column_name) FROM table_name;
         COUNT ( * ) Syntax:
        It returns the number of records in a table:
        SELECT COUNT ( * ) FROM table_name;
        COUNT ( DISTINCT column_name ) Syntax:
         It returns the number of distinct values of the specified column:
         SELECT COUNT ( DISTINCT column_name) FROM table_name;



MAX() Function:
        The MAX() function returns the largest value of the selected column.
        Syntax: SELECT MAX ( column_name ) FROM table_name;
        
MIN() Function:
         The MIN() Function returns the smallest value of the selected column.
         Syntax: SELECT MIN(column_name) FROM table_name;

SUM() Function:
         The SUM() Function returns the total sum of a numeric column.
          Syntax: SELECT SUM (column_name) FROM table_name;

LEN() Function: 
          The LEN() Function returns the length of the value in a text field.
           Syntax: SELECT LEN (column_name) FROM table_name;

HAVING Clause: The HAVING clause was added to SQL because the WHERE keyword could be used with aggregate functions.

GROUP BY Statement: The GROUP BY statement is used in conjunction with the aggregate functions to group the result_set by one or more columns.