How to generate a permutations or combinations of n rows in m columns?
Does anyone know how to create a table with m columns from table of n rows where the values in columns of each row represent a different combination or permutation of values from the original table?
For example the original table has 1 column (number_value) with 3 (n=3) rows:
1
2
3
The table which contains combinations (the order doesn't matter) of two values (m = 2) would be the following:
number1, number2
1,2
1,3
2,3
and the table of permutations would be the following:
number1, number2
1, 2
2, 1
1, 3
3, 1
2, 3
3, 2
The order of rows doesn't matter.
Thank you in advance!
Answer
Combinations:
SELECT T1.x, T2.x
FROM your_table T1
JOIN your_table T2
ON T1.x < T2.x
Permutations:
SELECT T1.x, T2.x
FROM your_table T1
JOIN your_table T2
ON T1.x != T2.x
I am assuming that the values in the original table are unique.
To generalize for larger values of m you need to add more joins.