Combination of all values (Oracle) - oracle

Question

I would like to get all possible combinations from a given data set, but even with empty elements (i.e. with a variable number of elements)
Example we have data and a simple query that displays them:
with t as ( select 1 as COL1, 2 as COL2, 3 as COL3 from dual )
select * from t;
or better example: (perhaps simple to solve)
select 1 as col from dual
union all
select 2 from dual
union all
select 3 from dual
Is it possible to create a query that displays the following result:
1 - -
1 2 -
1 2 3
1 3 -
1 3 2
2 - -
2 1 -
2 1 3
2 3 -
2 3 1
3 - -
3 1 -
3 1 2
3 2 -
3 2 1

Answer 1

One difficulty here is that you have the inputs in three columns and you want the results in three columns. Unless you are willing to use dynamic SQL (which is a separate topic, an advanced technique which is also a bad practice in most cases and has really nothing to do with your question about combinatorics) you will have to hard-code the number (and names) of columns in the query.
You could present the input data in rows rather than columns, and ask for the result in the format
row_num col_num val
------- ------- ---
1 1 1
1 2 -
1 3 -
(this mimics just your first output row) - and similar for all other rows - and then you would NOT have to hard-code the number of columns in the query; you could then easily adapt the code below to solve this more general problem.
I use SYS_CONNECT_BY_PATH below, which limits the number of columns (and the length of values in each column in the input); this can be avoided, but writing the query this way is fun.
I assumed the dash - in your desired output stands for NULL; if you actually want to show dashes, use NVL(..., -) in the final SELECT clause.
with
t (col1, col2, col3) as (
select 1 as col1, 2 as col2, 3 as col3 from dual
)
, prep (pth) as (
select sys_connect_by_path(val, '/') || '/'
from t
unpivot (val for col in (col1, col2, col3))
connect by nocycle prior col is not null
)
select to_number(substr(pth, instr(pth, '/', 1, 1) + 1,
instr(pth, '/', 1, 2) - instr(pth, '/', 1, 1) - 1)) col1,
to_number(substr(pth, instr(pth, '/', 1, 2) + 1,
instr(pth, '/', 1, 3) - instr(pth, '/', 1, 2) - 1)) col2,
to_number(substr(pth, instr(pth, '/', 1, 3) + 1,
instr(pth, '/', 1, 4) - instr(pth, '/', 1, 3) - 1)) col3
from prep
order by col1 nulls first, col2 nulls first, col3 nulls first
;
Output:
COL1 COL2 COL3
----- ----- -----
1
1 2
1 2 3
1 3
1 3 2
2
2 1
2 1 3
2 3
2 3 1
3
3 1
3 1 2
3 2
3 2 1