Select distinct values from multiple columns in same table

Question

I am trying to construct a single SQL statement that returns unique, non-null values from multiple columns all located in the same table.

 SELECT distinct tb         


        

Answer 1

try something like SubQuery:

SELECT derivedtable.NewColumn
FROM
(
    SELECT code_1 as NewColumn FROM tbl_data 
    UNION
    SELECT code_2 as NewColumn FROM tbl_data 
) derivedtable
WHERE derivedtable.NewColumn IS NOT NULL

The UNION already returns DISTINCT values from the combined query.