Why when using grouping_id with rollup the total will be level 3 instead of 2? - oracle

Question

Is there any logic behind the reason to why the total grouping when using rollup will be "lvl" 3?..
example:
col1
col2
id1
1
id1
2
id2
1
id2
2
id3
1
id3
2
In cube method this is understandable as
level 0 is the "basic values" (my own term)such as "col1-ID1","col2-id1", "col3-id1" etc..,
level 1 will be the subtotal row of each basic value which means subtotal of ID1(1)+ID1(2) --> id1(3) for example.
level 2 will be the total of each combination which means subtotal of 1's and subtotal of 2's , in this example: 1- subtotal will be 3 and 2-subtotal will be 6
and level 3 will be the grand total of them all, in this example: 9
my explanation might not make any sense:) .. sorry
Is there any reason behind that skipping from lvl 0/1 straight to 3? or its just the way it is?

Answer 1

Summary: The grand total will be in the grouping set with the id the maximum value of the grouping set which is equal to 2(number of columns in the grouping set) - 1. Therefore, with 2 columns being cubed, the grand total is in 22-1 = 3 and, with 3 columns being cubed, the grand total is in 23-1 = 7.
Given the query:
SELECT LISTAGG(col1, ',') WITHIN GROUP (ORDER BY col1) AS col1,
LISTAGG(col2, ',') WITHIN GROUP (ORDER BY col2) AS col2,
GROUPING_ID(col1, col2) AS grp
FROM table_name
GROUP BY CUBE(col1, col2)
ORDER BY grp, col1, col2
Which, for the sample data:
CREATE TABLE table_name (col1, col2) AS
SELECT 'id1', 1 FROM DUAL UNION ALL
SELECT 'id1', 2 FROM DUAL UNION ALL
SELECT 'id2', 1 FROM DUAL UNION ALL
SELECT 'id2', 2 FROM DUAL UNION ALL
SELECT 'id3', 1 FROM DUAL UNION ALL
SELECT 'id3', 2 FROM DUAL;
Outputs:
COL1
COL2
GRP
id1
1
0
id1
2
0
id2
1
0
id2
2
0
id3
1
0
id3
2
0
id1,id1
1,2
1
id2,id2
1,2
1
id3,id3
1,2
1
id1,id2,id3
1,1,1
2
id1,id2,id3
2,2,2
2
id1,id1,id2,id2,id3,id3
1,1,1,2,2,2
3
You can see that when GROUPING_ID(col1, col2) is:
The un-grouped value.
The value grouped by the first column in the grouping set (and there is one value for the first column and N values for the second column).
The value grouped by the second column in the grouping set (and there are M values for the first column and one value for the second column).
The value grouped by the both columns in the grouping set (and there are M values for the first column and N values for the second column giving N*M total values); which will give you the grand total.
If you had the sample data with 3 columns:
CREATE TABLE table2 (col1, col2, col3) AS
SELECT t1.COLUMN_VALUE,
t2.COLUMN_VALUE,
t3.COLUMN_VALUE
FROM TABLE(SYS.ODCIVARCHAR2LIST('id1', 'id2', 'id3')) t1
CROSS JOIN TABLE(SYS.ODCINUMBERLIST(1, 2)) t2
CROSS JOIN TABLE(SYS.ODCINUMBERLIST(3, 4)) t3;
Then using CUBE across 3 columns:
SELECT LISTAGG(col1, ',') WITHIN GROUP (ORDER BY col1) AS col1,
LISTAGG(col2, ',') WITHIN GROUP (ORDER BY col2) AS col2,
LISTAGG(col3, ',') WITHIN GROUP (ORDER BY col3) AS col3,
GROUPING_ID(col1, col2, col3) AS grp
FROM table2
GROUP BY CUBE(col1, col2, col3)
ORDER BY grp, col1, col2, col3
Outputs:
COL1
COL2
COL3
GRP
id1
1
3
0
id1
1
4
0
id1
2
3
0
id1
2
4
0
id2
1
3
0
id2
1
4
0
id2
2
3
0
id2
2
4
0
id3
1
3
0
id3
1
4
0
id3
2
3
0
id3
2
4
0
id1,id1
1,1
3,4
1
id1,id1
2,2
3,4
1
id2,id2
1,1
3,4
1
id2,id2
2,2
3,4
1
id3,id3
1,1
3,4
1
id3,id3
2,2
3,4
1
id1,id1
1,2
3,3
2
id1,id1
1,2
4,4
2
id2,id2
1,2
3,3
2
id2,id2
1,2
4,4
2
id3,id3
1,2
3,3
2
id3,id3
1,2
4,4
2
id1,id1,id1,id1
1,1,2,2
3,3,4,4
3
id2,id2,id2,id2
1,1,2,2
3,3,4,4
3
id3,id3,id3,id3
1,1,2,2
3,3,4,4
3
id1,id2,id3
1,1,1
3,3,3
4
id1,id2,id3
1,1,1
4,4,4
4
id1,id2,id3
2,2,2
3,3,3
4
id1,id2,id3
2,2,2
4,4,4
4
id1,id1,id2,id2,id3,id3
1,1,1,1,1,1
3,3,3,4,4,4
5
id1,id1,id2,id2,id3,id3
2,2,2,2,2,2
3,3,3,4,4,4
5
id1,id1,id2,id2,id3,id3
1,1,1,2,2,2
3,3,3,3,3,3
6
id1,id1,id2,id2,id3,id3
1,1,1,2,2,2
4,4,4,4,4,4
6
id1,id1,id1,id1,id2,id2,id2,id2,id3,id3,id3,id3
1,1,1,1,1,1,2,2,2,2,2,2
3,3,3,3,3,3,4,4,4,4,4,4
7
And will generate 23 = 8 levels (from 0 to 7) since there are all the possible combinations of grouping 3 columns and the grand-total will be in level 7; compared to 22 levels (0 to 3) when you are cubing 2 columns and the grand total is in level 3.
fiddle
Update
What I don't understand is why roll up is skipping level 2 straight to 3?
From the SELECT documentation:
ROLLUP
The ROLLUP operation in the simple_grouping_clause groups the selected rows based on the values of the first n, n-1, n-2, ... 0 expressions in the GROUP BY specification, and returns a single row of summary for each group.
[...]
CUBE
The CUBE operation in the simple_grouping_clause groups the selected rows based on the values of all possible combinations of expressions in the specification. It returns a single row of summary information for each group.
CUBE generates all possible grouping sets; ROLLUP generates groups of the first 1 column then with the first 2 columns, 3 columns, ..., up to n columns which is the same as the CUBE when the grouping sets are restricted to the 20-1, 21-1, 22-1, ..., 2n-1 (or more simply 0, 1, 3, 7, ... 2n-1).
This means that ROLLUP will skip the grouping set with id 2 as that is grouping only by the 2nd column and that is not "one of the first n, n-1, n-2, ... 0 expressions" in the GROUP BY specification.
fiddle