MYSQL output unique values

I have feed with entries where each entry (row) has 7 different categories.

I need to list all these categories, these are repeating often so I need list only of unique categories.

Categories are category1....category7 and there is at least 1 value for each row.

I found only this function http://cz.php.net/manual/en/function.array-unique.php and also GROUP BY but I am not sure how to combine it to get simple output of all categories in DB.

 

SELECT DISTINCT category_name from table_name WHERE ....

DISTINCT will fetch values which are unique...

 

But I would have to use something like:
SELECT DISTINCT category1,category2,...,category7 from table_name

I need to get all entries from categoryX and than select those uniques.

 

The output would be just text or another table with all unique categories from first table.

Anyone?

 

OK, could not wait and I spent whole day playing around and here is what I needed.

I hope it will be helpful to someone since I found few similar questions like this.