Difference between HAVING and WHERE Clause

HAVING specifies a search condition for a group or an aggregate function used in SELECT statement.

HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause.

A HAVING clause is like a WHERE clause, but applies only to groups as a whole, whereas the WHERE clause applies to individual rows. A query can contain both a WHERE clause and a HAVING clause. The WHERE clause is applied first to the individual rows in the tables . Only the rows that meet the conditions in the WHERE clause are grouped. The HAVING clause is then applied to the rows in the result set. Only the groups that meet the HAVING conditions appear in the query output. You can apply a HAVING clause only to columns that also appear in the GROUP BY clause or in an aggregate function. (Reference :BOL)

Example of HAVING and WHERE in one query:

SELECT titles.pub_id, AVG(titles.price)FROM titles INNER JOIN publishersON titles.pub_id = publishers.pub_idWHERE publishers.state = 'CA'GROUP BY titles.pub_idHAVING AVG(titles.price) > 10

Sometimes you can specify the same set of rows using either a WHERE clause or a HAVING clause. In such cases, one method is not more or less efficient than the other. The optimizer always automatically analyzes each statement you enter and selects an efficient means of executing it. It is best to use the syntax that most clearly describes the desired result. In general, that means eliminating undesired rows in earlier clauses.

Reference:http://blog.sqlauthority.com/2007/07/04/sql-server-definition-comparison-and-difference-between-having-and-where-clause/

GROUP BY vs ORDER BY : The ORDER BY keyword is used to sort the result-set by a specified column.The ORDER BY keyword sort the records in ascending order by default.If you want to sort the records in a descending order, you can use the DESC keyword..The GROUP BY statement is used in conjunction with the aggregate functions to group the result-set by one or more columns.
             The easiest explanation is that order by is doing the sorting of a table and the group by clause is used for aggregation of a field.

SELECT column_name(s)
FROM table_name
ORDER BY column_name(s) ASC|DESC

ex:SELECT * FROM Persons
ORDER BY LastName


SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name

ex :
SELECT Customer,SUM(OrderPrice) FROM Orders
GROUP BY Customer 

Refer : http://www.thetricky.net/mySQL/GROUP%20BY%20vs%20ORDER%20BY