Aggregate functions often need an added GROUP BY statement.
GROUP BY statement is used in conjunction with the aggregate functions to group the result-set by one or more columns.
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;
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 |
And a selection from the "Shippers" table:
| ShipperID | ShipperName | Phone |
|---|---|---|
| 1 | Speedy Express | (503) 555-9831 |
| 2 | United Package | (503) 555-3199 |
| 3 | Federal Shipping | (503) 555-9931 |
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 the number of orders sent by each shipper.
following SQL statement counts as orders grouped by shippers:
SELECT Shippers.ShipperName,COUNT(Orders.OrderID) AS NumberOfOrders FROM
Orders
LEFT JOIN Shippers
ON Orders.ShipperID=Shippers.ShipperID
GROUP BY ShipperName;
We can also use the GROUP BY statement on more than one column, like this:
SELECT Shippers.ShipperName, Employees.LastName,
COUNT(Orders.OrderID) AS
NumberOfOrders
FROM ((Orders
INNER JOIN Shippers
ON
Orders.ShipperID=Shippers.ShipperID)
INNER JOIN Employees
ON
Orders.EmployeeID=Employees.EmployeeID)
GROUP BY ShipperName,LastName;