How to Get Distinct More than one Column Rows in one Column

Question

I want 5 column's Rows in one Column And Unique Rows in That Column.

SELECT DISTINCT date,accept_date, question_date,success_date
FROM user_presentation;

I Got Rows as:

date       accept_date question_date success_date
2018-12-07 2018-11-23  2018-12-21    2019-01-04
2018-12-21 2018-12-07  2019-01-04    2019-01-18
2019-01-04 2018-12-21  2019-01-18    2019-02-01
2019-01-18 2019-01-04  2019-02-01    2019-02-15
2019-02-01 2019-01-18  2019-02-15    2019-03-01

I want All Rows in one column But Only Unique Values

Answer 1

SELECT DISTINCT val
  FROM 
     ( SELECT 'date' date_type 
            , date val
         FROM user_presentation
        UNION 
       SELECT 'accept_date' 
            , accept_date 
         FROM user_presentation
        UNION 
       SELECT 'question_date'
            , question_date  
         FROM user_presentation
        UNION 
       SELECT 'success_date'
            , success_date
         FROM user_presentation
     ) normalized;