MySQL SELECT DISTINCT multiple columns

Let's say I have columns a, b c, d in a table in a MySQL database. What I'm trying to do is to select the distinct values of ALL of these 4 columns in my table (only the distinct values). I tried stuff like:

SELECT DISTINCT a,b,c,d FROM my_table;
SELECT DISTINCT a,b,c,d FROM my_table GROUP BY a,b,c,d;

None of those worked. Can anybody help out here?

Thank you

NOTE I want the distinct values of the columns a, b, c d separately. Not the distinct combination of values

I know that the question is too old, anyway:

SELECT a, b
FROM mytable
GROUP BY a, b;

This will give your all the combinations.

can this help?

select 
(SELECT group_concat(DISTINCT a) FROM my_table) as a,
(SELECT group_concat(DISTINCT b) FROM my_table) as b,
(SELECT group_concat(DISTINCT c) FROM my_table) as c,
(SELECT group_concat(DISTINCT d) FROM my_table) as d

Taking a guess at the results you want so maybe this is the query you want then

SELECT DISTINCT a FROM my_table
UNION 
SELECT DISTINCT b FROM my_table
UNION
SELECT DISTINCT c FROM my_table
UNION
SELECT DISTINCT d FROM my_table

Another simple way to do it is with concat()

SELECT DISTINCT(CONCAT(a,b)) AS cc FROM my_table GROUP BY (cc);

Both your queries are correct and should give you the right answer.

I would suggest the following query to troubleshoot your problem.

SELECT DISTINCT a,b,c,d,count(*) Count FROM my_table GROUP BY a,b,c,d
order by count(*) desc

That is add count(*) field. This will give you idea how many rows were eliminated using the group command.

This will give DISTINCT values across all the columns:

SELECT DISTINCT value
FROM (
	SELECT DISTINCT a AS value FROM my_table
	UNION SELECT DISTINCT b AS value FROM my_table
	UNION SELECT DISTINCT c AS value FROM my_table
) AS derived