Selecting all bi-directional connection in a Table

Question

I have a tricky question and I couldn't really find an answer that would suit my database schema.

Let's say I have the following table:

CONNECTIONS

And the following columns

connID, StartNode, EndNode

and let's say you can have bi-directional connection from StartNode to EndNode

StartNode -> EndNode

EndNode -> StartNode

And it's represented in a table as the following:

CONNECTIONS
|connID     |  StartNode   |  EndNode   |
|   1       |    9         |    10      |
|   2       |    10        |    9       |
|   3       |    9         |    11      |

My question is: What SQL statement would help me in retrieving all the connections that are bi-directional (and in this case the first 2 connections above with connId 1 and 2?

Answer 1

You can join your table to itself putting the equality between start and endnote as in the following example

select A.* from connections A
inner join connections B on (A.startnode=B.endnode and A.endnode = B.startnode)