MySQL SELECT DISTINCT multiple columns

Question

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 tabl

Answer 1

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