SQLServer the GROUP BY statement

GROUP BY statement

GROUP BY statement is used in conjunction with aggregate functions, grouping result set according to 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 Examples

We have the following "Orders" table:

O_Id	OrderDate	OrderPrice	Customer
1	2008/12/29	1000	Bush
2	2008/11/23	1600	Carter
3	2008/10/05	700	Bush
4	2008/09/28	300	Bush
5	2008/08/06	2000	Adams
6	2008/07/21	100	Carter

Now, we want to find the total amount per customer (total order).

We want to use the GROUP BY statement to customers combined.

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)
Bush	2000
Carter	1700
Adams	2000

Great bar, right?

Let's look at what happens if you omit GROUP BY occur:

	SELECT Customer,SUM(OrderPrice) FROM Orders

The result set will look like this:

Customer	SUM(OrderPrice)
Bush	5700
Carter	5700
Bush	5700
Bush	5700
Adams	5700
Carter	5700

The above result set is not what we need.

So why not use this SELECT statement above it? Explained as follows: The above SELECT statement specifies two (Customer and SUM (OrderPrice)). "SUM (OrderPrice)" returns a single value ( "OrderPrice" column total), the "Customer" return value 6 (each corresponding "Orders" in each row in the table). Therefore, we can not get the correct result. However, you have seen, GROUP BY statement solves this problem.

More than one GROUP BY column

We can also apply for more than one column GROUP BY statement, like this:

	SELECT Customer,OrderDate,SUM(OrderPrice) FROM Orders
GROUP BY Customer,OrderDate
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