如何在SQL中从对中查找所有三元组？

Question

Assume that I use SQLite query syntax.

I have a table of distances between points:

CREATE TABLE d (
    p1 INT,
    p2 INT,
    distance REAL
);

If I know distance between p1 and p2 then I known distance between p2 and p1 - both rows are in table.

I want to create a view ( select query) for all unique triples of points, so that I know the distances between all three of them pairwise.

I tried the following:

CREATE VIEW triple as
    select d1.p1 as p1, d2.p1 as p2, d3.p1 as p3
    from d as d1, d as d2, d as d3
    where d1.p1=d3.p2 and d1.p2=d2.p1 and d2.p2=d3.p1;

But I don't know how to get rid of transpositions like:

1 | 2 | 3
2 | 3 | 1
3 | 1 | 2

What is a fast and correct select query for my view?

Answer 1

If you have all pairs, you can use pair-wise comparisons. Here is one method:

CREATE VIEW triple as
    select d1.p1 as p1, d2.p1 as p2, d3.p1 as p3
    from d d1 join
         d d2
         on d1.p2 = d2.p1 join
         d as d3
         on d1.p1 = d3.p2 and d2.p2 = d3.p1
    where d1.p1 < d1.p2 and d3.p1 < d3.p2;