SQL GROUP BY Statement

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# 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_Id | OrderDate | OrderPrice | Customer
--- | --- | --- | ---
1 | 2008/11/12 | 1000 | Hansen
2 | 2008/10/23 | 1600 | Nilsen
3 | 2008/09/02 | 700 | Hansen
4 | 2008/09/03 | 300 | Hansen
5 | 2008/08/30 | 2000 | Jensen
6 | 2008/10/04 | 100 | Nilsen

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:

Customer | SUM(OrderPrice)
--- | ---
Hansen | 2000
Nilsen | 1700
Jensen | 2000

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:

Customer | SUM(OrderPrice)
--- | ---
Hansen | 5700
Nilsen | 5700
Hansen | 5700
Hansen | 5700
Jensen | 5700
Nilsen | 5700

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

From http://www.w3schools.com (Copyright Refsnes Data)