SQL GROUP BY Statement

Aggregate functions often need an added GROUP BY statement.
The GROUP BY Statement

The GROUP BY statement is used in conjunction with the aggregate functions to group the result-set by one or more columns.

SQL GROUP BY Syntax
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
SQL GROUP BY Example
We have the following "Orders" table:
O_IdOrderDateOrderPriceCustomer
12008/11/121000Hansen
22008/10/231600Nilsen
32008/09/02700Hansen
42008/09/03300Hansen
52008/08/302000Jensen
62008/10/04100Nilsen

Now we want to find the total sum (total order) of each customer.

We will have to use the GROUP BY statement to group the customers.

We use the following SQL statement:
SELECT Customer,SUM(OrderPrice) FROM Orders
GROUP BY Customer
The result-set will look like this:
CustomerSUM(OrderPrice)
Hansen2000
Nilsen1700
Jensen2000

Nice! Isn't it? :)

Let's see what happens if we omit the GROUP BY statement:
SELECT Customer,SUM(OrderPrice) FROM Orders
The result-set will look like this:
CustomerSUM(OrderPrice)
Hansen5700
Nilsen5700
Hansen5700
Hansen5700
Jensen5700
Nilsen5700

The result-set above is not what we wanted.

Explanation of why the above SELECT statement cannot be used: The SELECT statement above has two columns specified (Customer and SUM(OrderPrice). The "SUM(OrderPrice)" returns a single value (that is the total sum of the "OrderPrice" column), while "Customer" returns 6 values (one value for each row in the "Orders" table). This will therefore not give us the correct result. However, you have seen that the GROUP BY statement solves this problem.
GROUP BY More Than One Column

We can also use the GROUP BY statement on more than one column, like this:
SELECT Customer,OrderDate,SUM(OrderPrice) FROM Orders
GROUP BY Customer,OrderDate

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