SQL - Find all possible combination

I need help finding all possible combination of value in database, for Example i have this table:

ITEM_SET           Support  
I1               0.244999999
I2               0.274999999     
I3               0.258333333    
I4               0.103333333  

i need to find all possible combination like this :

I1,I2    I1,I2,I3    I1,I2,I3,I4
I1,I3    I1,I2,I4
I1,I4    I1,I3,I4
I2,I3    I2,I3,I4
I2,I4 
I3,I4

*Please note that this formatting meant only to help reading, as what开发者_运维知识库 i need is only a list of the possible combination like this Table:

ITEMSET
I1               
I2                    
I3                 
I4  
I1,I2    
I1,I3    
I1,I4    
I2,I3    
I2,I4 
I3,I4
I1,I2,I3 
I1,I2,I4 
I1,I3,I4 
I2,I3,I4   
I1,I2,I3,I4

What you're suggesting is an n! combination of all of the elements for lengths 1-n. Ignoring the possibility of using a code generator to create your elements, you could do something like this for each combination (in MySQL):

One item:

SELECT item from ITEM_SET;

Two items:

SELECT one.item,two.item from ITEM_SET as one, ITEM_SET as two where one.item != two.item;

Three items:

SELECT one.item,two.item,three.item from ITEM_SET as one, ITEM_SET as two, ITEM_SET as three where one.item != two.item and one.item != three.item and two.item != three.item;

Rinse and repeat. To be pedantic, I define ITEM_SET as my table name and item as my attribute, which is a more meaningful table composition.

This and the related question are code smells to me, though. If you're walking all permutations of elements programmatically for all candidate answers, there's likely a much simpler algorithm to solve your problem. Given your other question is directly related to this one, perhaps you can offer more background information?

One of the most simple algorithms for generating combinations is bit counting.
Pseudo-code

N items, indexed 1-N

for i=1 to 2^N-1 
   for each bit in i
      if bit is set, output item[i]

Example for N=4:

N = 4, 2^4 = 16
i = 1:  binary = 00000001 -> output I1
i = 2:  binary = 00000010 -> output I2
i = 3:  binary = 00000011 -> output I1, I2
i = 4:  binary = 00000100 -> output I3
i = 6:  binary = 00000101 -> output I1, I3
i = 7:  binary = 00000111 -> output I1, I2, I3
i = 8:  binary = 00001000 -> output I4
i = 9:  binary = 00001001 -> output I1, I4
i = 10: binary = 00001010 -> output I2, I4
i = 11: binary = 00001011 -> output I1, I2, I4
i = 12: binary = 00001100 -> output I3, I4
i = 13: binary = 00001011 -> output I1, I2, I4
i = 14: binary = 00001110 -> output I2, I2, I4
i = 15: binary = 00001111 -> output I1, I2, I3, I4