Question

I’m trying to get data where order doesn’t matter with unique ids. So simply my query would be

SELECT DISTINCT id1, id2 FROM messages ORDER BY date

If i have a database with the following data:

id1 | id2 | date
 5  |  6  |  1/2/2011
 6  |  5  |  1/1/2011

I would only need to load the column with the newest date because the ids are the same 2 people. Really i have to load ids where one of the ids is yours so my real query right now is

SELECT DISTINCT userid_1, userid_2 
  FROM messages 
    WHERE userid_2=$dbid 
    OR userid_1=$dbid 
ORDER BY date

and i get a result as [6 5] [5 9] [9 5] [5 15] [5 6] [5 17]

Results 2 and 3 are the same and 1 and 5 are the same. Really only 4 results should be queried. Thanks!

Answer 1

One option is:

SELECT DISTINCT 
if(userid_1 >= userid_2,userid_1, userid_2) AS 'id1', 
if(userid_1 >= userid_2,userid_2, userid_1) AS 'id2'

FROM messages 
WHERE userid_2=$dbid OR userid_1=$dbid ORDER BY date

1. This query shows two fields for each record.
2. The first field will be userid1 if it is bigger or equal to userid2, else will show userid2
3. The second field has the opposite logic
4. This makes sure that two similar sets of results will alwas be ordered the same way, so the DISTINCT will regard it as same.