034 SQL HAVING

The W3Schools "SQL HAVING Clause" page explains the purpose and use of the SQL HAVING clause:

Purpose: The HAVING clause is used to filter groups in SQL queries, especially when you want to apply conditions to aggregated results (like counts or sums). Unlike the WHERE clause, HAVING works with aggregate functions such as COUNT, SUM, AVG, etc.

Syntax Example:

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
HAVING condition
ORDER BY column_name(s);

Key Details:
- The page provides sample tables from the "Northwind" database to demonstrate.
- Examples show how to use HAVING to filter groups, such as countries with more than five customers or employees with more than a certain number of orders.
- Example SQL:
```
SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
HAVING COUNT(CustomerID) > 5;
```
- This query lists countries with more than five customers.
Quiz: The page asks: What is the purpose of the HAVING clause? The correct answer is: "To filter groups based on an aggregate condition after grouping".[1]

Overall, the HAVING clause is essential for filtering data that results from grouping and aggregation, a task not possible with WHERE alone.

SQL HAVING Clause

The SQL HAVING Clause

The HAVING clause was added to SQL because the WHERE keyword cannot be used with aggregate functions.

HAVING Syntax

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
HAVING condition
ORDER BY column_name(s);

Demo Database

Below is a selection from the "Customers" table in the Northwind sample database:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SQL HAVING Examples

The following SQL statement lists the number of customers in each country. Only include countries with more than 5 customers:

SELECT COUNT(CustomerID), Country  
FROM Customers  
GROUP BY Country  
HAVING COUNT(CustomerID) > 5;

The following SQL statement lists the number of customers in each country, sorted high to low (Only include countries with more than 5 customers):

SELECT COUNT(CustomerID), Country  
FROM Customers  
GROUP BY Country  
HAVING COUNT(CustomerID) > 5  
ORDER BY COUNT(CustomerID) DESC;

Demo Database

Below is a selection from the "Orders" table in the Northwind sample database:

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10248	90	5	1996-07-04	3
10249	81	6	1996-07-05	1
10250	34	4	1996-07-08	2

And a selection from the "Employees" table:

EmployeeID	LastName	FirstName	BirthDate	Photo	Notes
1	Davolio	Nancy	1968-12-08	EmpID1.pic	Education includes a BA....
2	Fuller	Andrew	1952-02-19	EmpID2.pic	Andrew received his BTS....
3	Leverling	Janet	1963-08-30	EmpID3.pic	Janet has a BS degree....

More HAVING Examples

The following SQL statement lists the employees that have registered more than 10 orders:

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders  
FROM (Orders  
INNER JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID)  
GROUP BY LastName  
HAVING COUNT(Orders.OrderID) > 10;

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders  
FROM Orders  
INNER JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID  
WHERE LastName = 'Davolio' OR LastName = 'Fuller'  
GROUP BY LastName  
HAVING COUNT(Orders.OrderID) > 25;