How to get all distinct words in a column

I have a table with a text column that can contain a single word, more words separated by a space or a NULL value. I need to get all the distinct words in that column (words non column entries).

For getting all the distinct column entries I use

SELECT DISTINCT(col_name) AS col_name FROM table_name ORDER BY col_name ASC

but I don’t know how to get all the distinct words.

SELECT DISTINCT      
  SUBSTRING_INDEX(SUBSTRING_INDEX(table_name.col_name, ' ', numbers.n), ' ', -1) col_name
FROM
  (SELECT 1 n UNION ALL SELECT 2
   UNION ALL SELECT 3 UNION ALL SELECT 4) numbers INNER JOIN table_name
  ON CHAR_LENGTH(table_name.col_name)
     -CHAR_LENGTH(REPLACE(table_name.col_name, ' ', ''))>=numbers.n-1
ORDER BY
  col_name;

You can see it here: http://sqlfiddle.com/#!2/b5be5/2/0

reference: https://stackoverflow.com/questions/17942508/sql-split-values-to-multiple-rows