SQL GROUP BY statement, sqlgroupby statement

Source: Internet
Author: User

SQL GROUP BY statement, sqlgroupby statement

A group by statement is often required for Aggregate functions (such as SUM.

Group by statement

The group by statement is used in combination with the aggregate function to GROUP result sets based on one or more columns.

SQL GROUP BY syntax
SELECT column_name, aggregate_function(column_name)FROM table_nameWHERE column_name operator valueGROUP BY column_name
SQL GROUP BY instance

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 (total order) for each customer ).

We want to use the group by statement to combine the customer.

We use the following SQL statement:

SELECT Customer,SUM(OrderPrice) FROM OrdersGROUP BY Customer

The result set is similar to the following:

Customer SUM (OrderPrice)
Bush 2000
Carter 1700
Adams 2000

 

Great, right?

Let's take a look at what will happen if group by is omitted:

SELECT Customer,SUM(OrderPrice) FROM Orders

The result set is similar to the following:

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 can't I use the preceding SELECT statement? The preceding SELECT statement specifies two columns (Customer and SUM (OrderPrice )). "SUM (OrderPrice)" returns a separate value (total of the "OrderPrice" column ), "Customer" returns six values (each value corresponds to each row in the "Orders" table ). Therefore, we cannot get the correct results. However, you can see that the group by statement solves this problem.

Group by more than one column

You can also apply the group by statement to more than one column, as shown in the following figure:

SELECT Customer,OrderDate,SUM(OrderPrice) FROM OrdersGROUP BY Customer,OrderDate

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