T-SQL：表中的逻辑唯一表行

Question

I have simple table containing information about bidirectional links between elements. Table looks like this:

link_id | link_side_a_element_id | link_side_b_element_id
---------------------------------------------------------
      1 |                    100 |                    200
      2 |                    200 |                    100
      3 |                    300 |                    400
      4 |                    400 |                    300

All I want is to select unique links between elements, no matter which element is sideA or sideB. For example, link between 100 and 200 is the same link as between 200 and 100, and only one record should be returned by query, something like:

link_side_a_element_id | link_side_b_element_id
-----------------------------------------------
                   100 |                    200
                   300 |                    400

This will be enough for me. But stuck on write proper select DML... Now I do it in Java, fetching all data to Set with proper hashCode() and equals() methods. This is working for me for now, but want to get data right in SQL Management Studio... How to achieve this?

Answer 1

Just pick an order that the two items should appear in (eg always list the lowest/earliest first) and then run a DISTINCT on that:

SELECT DISTINCT
   CASE WHEN link_side_a_element_id < link_side_b_element_id THEN link_side_a_element_id
        ELSE link_side_b_element_id END as a,
   CASE WHEN link_side_a_element_id < link_side_b_element_id THEN link_side_b_element_id
        ELSE link_side_a_element_id END as b
FROM
    table