Apart from grouping and aggregation, PostgreSQL provides another way to perform computations based on the values of several records. It can be done by using the window functions. Grouping and aggregation implies the output of a single record for every group of input records. Window functions can do similar things, but they are executed for every record, and the number of records in the output and the input is the same:
GROUP BY and Window functions
In the preceding diagram, the rectangles represent the records of a table. Let's assume that the color of the rectangles indicates the value of a field, used to group the records. When GROUP BY is used in a query, each distinct value of that field will create a group, and each group will become a single record in the results of the query. That was explained in Chapter 5, SQL Language. Window functions can access the values of other records of the same group (which is called a partition in this case), although the number of records...