Programming the Perl DBI

Grouping data

The ability to group items of data is very useful when attempting to make summarized reports. SQL features a clause called GROUP BY that allows you to group rows that share a common set of values and apply group functions to them.

A good example of this operation is where you want to total the values contained within a column in a table. In this instance, you would use the sum( ) grouping function in the following way to calculate the total value of orders taken on a given date:

SELECT order_date, sum( net ), sum( vat ), sum( total )
FROM sales
GROUP BY order_date

As with ORDER BY, groupings can be chained together in a comma-separated list to create complex subgroupings of columns.