2. Aggregate Functions in SQL (Min, Max, Sum, Avg, Count)

MIN	returns the smallest value in a given column
MAX	returns the largest value in a given column
SUM	returns the sum of the numeric values in a given column
AVG	returns the average value of a given column
COUNT	returns the total number of values in a given column
COUNT(*)	returns the number of rows in a table

Aggregate functions are used to compute against a "returned column of numeric data" from your SELECT statement. They basically summarize the results of a particular column of selected data. We are covering these here since they are required by the next topic, "GROUP BY". Although they are required for the "GROUP BY" clause, these functions can be used without the "GROUP BY" clause. 

For example:

SELECT AVG(salary)

FROM employee;

This statement will return a single result which contains the average value of everything returned in the salary column from the employee table.

Another example:

SELECT AVG(salary)

FROM employee;

WHERE title = 'Programmer';

This statement will return the average salary for all employees whose title is equal to 'Programmer'

Example:

SELECT Count(*)

FROM employees;

This particular statement is slightly different from the other aggregate functions since there isn't a column supplied to the count function. This statement will return the number of rows in the employees table.