Getting a set with unique column values from MySQL

Question

Say that I have a set with columns like this

user_id | username | updated_at | data...

user_id and username are not unique in the set, so you can have something like this

user_id | username | updated_at | data...
------------------------------------------
   1    |   test   |  140****** | ...  
   4    |   test2  |  140****** | ...  
   1    |   test   |  139****** | ...  
   7    |   meh    |  140****** | ...   

But I would like to remove the duplicate occurrences; I tried GROUP BY but it gives me something unexpected as a lot of items are getting removed (I guess they appears later in the set as the query has a LIMIT in it).

Answer 1

If you need to select all the data - first of all u should decide how to get the updated_at and data columns.

In case you want to have the data concatenated and you want to have the latest updated_at you should do

SELECT user_id, username, max(updated_at), group_concat(data separator ',')
FROM table_name
GROUP BY user_id, username
ORDER BY user_id, username
LIMIT X

in that case your data will be ordered by user_id, and username

Note: it is not clear from your question if you want to remove the data from the table itself or only from the result set.

Answer 2

Is this what you want?

select distinct user_id, username
from table t;