Having Clause in SQL Server

 Specifies a search condition for a group or an aggregate. HAVING can be used only with the SELECT statement. HAVING is typically used with a GROUP BY clause. When GROUP BY is not used, there is an implicit single, aggregated group.

Because the WHERE clause is evaluated before GROUP BY, you cannot use WHERE to pare down results of the grouping (typically an aggregate function, such as COUNT(*)). To meet this need, the HAVING clause can be used.

[Logical Processing Order of SELECT statement]

Let's see the below example for more clarity.

DECLARE @orders TABLE(OrderID INT, Name NVARCHAR(100))
INSERT INTO @orders VALUES
( 1, 'Matt' ),
( 2, 'John' ),
( 3, 'Matt' ),
( 4, 'Luke' ),
( 5, 'John' ),
( 6, 'Luke' ),
( 7, 'John' ),
( 8, 'John' ),
( 9, 'Luke' ),
( 10, 'John' ),
( 11, 'Luke' )

If we want to get the number of orders each person has placed, we would use

SELECT Name, COUNT(*) AS 'Orders'
FROM @orders
GROUP BY Name

Output:

Name	Orders
Matt	2
John	5
Luke	4

SELECT Name, COUNT(*) AS 'Orders'
FROM @orders
GROUP BY Name
HAVING COUNT(*) > 2

Output:

Name	Orders
John	5
Luke	4

Note that, much like GROUP BY, the columns put in HAVING must exactly match their counterparts in the SELECT

statement. If in the above example we had instead said

SELECT Name, COUNT(DISTINCT OrderID)

our HAVING clause would have to say

HAVING COUNT(DISTINCT OrderID) > 2