Aggregate functions often need an added 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.
We have the following "Purchases" table:
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:
The result-set will look like this:
Nice! Isn't it? :)
Let's see what happens if we omit the GROUP BY statement:
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 "Purchases" table). This will
therefore not give us the correct result. However, you have seen that the GROUP BY statement
solves this problem.
We can also use the GROUP BY statement on more than one column, like this:
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