If you want to follow along you can use this query for getting the same table as the exercise

SQL
1create table bricks (
2  brick_id integer,
3  colour   varchar2(10),
4  shape    varchar2(10),
5  weight   integer
6);
7
8insert into bricks values ( 1, 'blue', 'cube', 1 );
9insert into bricks values ( 2, 'blue', 'pyramid', 2 );
10insert into bricks values ( 3, 'red', 'cube', 1 );
11insert into bricks values ( 4, 'red', 'cube', 2 );
12insert into bricks values ( 5, 'red', 'pyramid', 3 );
13insert into bricks values ( 6, 'green', 'pyramid', 1 );
14
15commit;

Group by functions let us analyze information on a table with a single query, for example using the following table.

BRICK_ID	COLOUR	SHAPE	WEIGHT
1	blue	cube	1
2	blue	pyramid	2
3	red	cube	1
4	red	cube	2
5	red	pyramid	3
6	green	pyramid	1

We can group by colour and get the sum of the weight of all the figures of each colour

SQL
1SELECT colour, SUM(weight)
2FROM bricks
3GROUP BY colour;

The result is:

COLOUR	SUM(WEIGHT)
red	6
green	1
blue	3

But then imagine that you want to group by but now by shape but also you want to see the group by colour so we can use GROUPING SETS()

SQL
1SELECT colour, shape, SUM(weight)
2FROM bricks
3GROUP BY GROUPING SETS(colour, shape);

So now with these query we obtain the following information.

COLOUR	SHAPE	SUM(WEIGHT)
-	cube	4
-	pyramid	6
green	-	1
blue	-	3
red	-	6