Introducing grouping sets

Every advanced user of SQL should be familiar with the GROUP BY and HAVING clauses. But are they also aware of CUBE, ROLLUP, and GROUPING SETS? If not, this chapter is a must-read. What is the basic idea behind a grouping set? Basically, the concept is simple: by using a grouping set, you can combine various aggregations into a single query. The main advantage is that you have to read data only once while producing many different aggregation sets at the same time.

Loading some sample data

To make this chapter a pleasant experience for you, we will compile some sample data that has been taken from the BP energy report, which can be found at https://www.bp.com/en/global/corporate/energy-economics.html.

Here is the data structure that will be used:

test=# CREATE TABLE t_oil (
    region        TEXT,
    country       TEXT,
  ...