Answer in one line is : 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.
Though the HAVING clause specifies a condition that is
similar to the purpose of a WHERE clause, the two clauses
are not interchangeable. Listed below are some differences
to help distinguish between the two:
1. The WHERE clause specifies the criteria which individual
records must meet to be selcted by a query. It can be used
without the GROUP BY clause. The HAVING clause cannot be
used without the GROUP BY clause.
2. The WHERE clause selects rows before grouping. The
HAVING clause selects rows after grouping.
3. The WHERE clause cannot contain aggregate functions. The
HAVING clause can contain aggregate functions.
The HAVING clause allows you to filter the results of
aggregate functions,
such as COUNT() or AVG() or SUM(), or MAX() or MIN(), just
to name a few.
HAVING provides you a means to filter these results in the
same query,
as opposed to saving the results of a WHERE clause SQL
statement to a temporary table
and running another query on the temporary table results to
extract the same results.
Follow The Below Example This Will Clear Up
Go
Create table Test_Where_Having
(id int identity,[Name] varchar(20),[Age] int,Amount int)
Go
insert into Test_Where_Having
select 'Abc',20,100
union all
select 'Def',30,100
union all
select 'Ghi',52,500
union all
select 'Jkl',30,80
union all
select 'Mno',40,600
union all
select 'Pqr',60,500
union all
select 'Pqr',60,500
union all
select 'Abc',20,500
Go
--They both are use to exclude rows from the resultset,
--but "where" is to filter the original set
--and "having" is in case you are grouping.
select [Name],[Age] from Test_Where_Having
where Age>30
--Output
Name Age
-------------------- -----------
Ghi 52
Mno 40
Pqr 60
Pqr 60
select [Name],[Age],Sum(Amount) As Total from
Test_Where_Having
group by [Name],[Age]
having Sum(Amount)>400
--Output
Name Age Total
-------------------- ----------- -----------
Abc 20 600
Mno 40 600
Ghi 52 500
Pqr 60 1000
--For Below Case These Queries Are Same As per Performance
(Cost Of Query) And result basis
select [Name],[Age],Sum(Amount) As Total from
Test_Where_Having
where Age>30
group by [Name],[Age]
select [Name],[Age],Sum(Amount) As Total from
Test_Where_Having
group by [Name],[Age]
having Age>30
--For Below Case These Queries Are Same As per Performance
(Cost Of Query) And result basis
select [Name],[Age] from Test_Where_Having
where Age>30
group by [Name],[Age]
select [Name],[Age] from Test_Where_Having
group by [Name],[Age]
having Age>30
--WHERE is used to filter rows. HAVING is usually used to
filter rows after performing an aggregation.
--Below Query is incorrect While using aggregate with where
clause
select [Name],[Age],Sum(Amount) As Total from
Test_Where_Having
where Sum(Amount)>30
group by [Name],[Age]
--U can use the above with Having
select [Name],[Age],Sum(Amount) As Total from
Test_Where_Having
group by [Name],[Age]
having Sum(Amount)>30
--Output
Name Age Total
-------------------- ----------- -----------
Abc 20 600
Def 30 100
Jkl 30 80
Mno 40 600
Ghi 52 500
Pqr 60 1000
--You can't use HAVING unless you also use GROUP BY.
--Ie below Query is incorrect
select [Name],[Age],Sum(Amount) As Total from
Test_Where_Having
having Sum(Amount)>30
--One limitation when you use the HAVING clause as compare
to WHERE clause.
-- Having clause only supports the Grouped Columns &
Aggregation filter..
--Where there is a column level filter then always use the
Where clause,
--Use Having clause only for Aggregation filter.
--For Example Below U cant use Amount column in Having
Clause because it is not in grouped columns
select [Name],[Age] from Test_Where_Having
group by [Name],[Age]
having Amount>30
--For The Above Case U can Use Where Clause
select [Name],[Age] from Test_Where_Having
where Amount>30
group by [Name],[Age]