I have a table with 3 VARCHARs in 3 different columns:

         user    owner   initiatorUser
------------------------------------
row1     UserA,  UserC,  UserC
row2     UserB,  UserC,  UserA

Now I need every occurrence in all rows and in all the 3 columns as distinct VARCHAR result set or, if not possible, as a CSV.

UserA
UserB  
UserC

I know that was discussed quite often here, but none of the solutions applies for me. I experimented with MySQL's GROUP_CONCAT, but couldn't get a fitting result.