Tutorial :2-column distinct?



Question:

I'm trying to select two distinct numbers id1 and id2 from the following table:

tb_table1( bigint id1 bigint id2 bigint userid)

if I do

select distinct id1, id2 from tb_table1   

I'll get, for example, two rows, (111, 222) and (222,111).

I only want one of those rows since I don't care which column, id1, or id2 that the result gets returned in. Basically, I want distinct pairs where order doesn't matter.

Thoughts? Thanks in advance.


Solution:1

It would be remiss of me to not point out that this suggests your table is not quite as normalized as it should be - what will you do when users acquire a third id? But anyway.

Using the fact that UNION (as opposed to UNION ALL) will automatically de-duplicate, you could do

SELECT id1, id2 FROM tb_table1 WHERE id1 < id2  UNION  SELECT id2, id1 FROM tb_table1 WHERE NOT id1 < id2  


Solution:2

I think there is a better solution but this should work:

select t1.id1, t2.id2   from tb_table1 t1  inner join tb_table1 t2 on t1.id1 = t2.id2 and t1.id2 = t2.id1  where t2.id1 is null  


Solution:3

would something like this be any help...

SELECT DISTINCT CASE WHEN id1 < id2 THEN id1 ELSE id2,                  CASE WHEN id1 > id2 THEN id1 ELSE id2      FROM tb_table1;  

(Having seen AakashM's solution, though, I think I like it better).


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