combinations (not permutations) from cross join in sql

问题

If I have a table that I'd like to cross join to itself, how can I remove the duplicate rows? Or to put it another way, how can I do a "order doesn't matter" cross join?

So for example, if I have a table T:

field |
-------
   A  |
   B  |
   C  |

and I cross join to itself so that i don't get the A | A rows

T as t1
cross join
T as t2
  on t1.field != t2.field

I would get the following:

field | field
------+-------
  A   |   B
  A   |   C
  B   |   A
  B   |   C
  C   |   A
  C   |   B

However, to me A, B is the same as B, A.

Is there a good way to remove these duplicates? In other words, I want the combinations not the permutations.

回答1:

T as t1
inner join
T as t2
  on t1.field < t2.field

FWIW, you can just use INNER JOIN for this, it's not strictly a CROSS JOIN. MySQL (and perhaps some other RDBMS) treats these two types of join as identical, but in ANSI SQL, a cross join has no join condition -- it's a deliberate Cartesian product.

来源：https://stackoverflow.com/questions/7112513/combinations-not-permutations-from-cross-join-in-sql