HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions.
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value;
このチュートリアルでは、よく知られた Northwind サンプルデータベースを使用します。
次は、"Orders" テーブルからの抜粋です：
| 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 |
および "Employees" テーブルからの抜粋です：
| 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.... |
Now we want to find if any of the employees has registered more than 10 orders.
次の SQL 文を使用します：
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;
Now we want to find if the employees "Davolio" or "Fuller" have registered more than 25 orders.
We add an ordinary WHERE clause to the SQL statement:
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;