How to execute UNION without sorting? (SQL)

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I notice this question gets quite a lot of views so I’ll first address a question you didn’t ask!

Regarding the title. To achieve a “Sql Union All with “distinct”” then simply replace UNION ALL with UNION. This has the effect of removing duplicates.

For your specific question, given the clarification “The first query should have “priority”, so duplicates should be removed from bottom” you can use

SELECT col1,
       col2,
       MIN(grp) AS source_group
FROM   (SELECT 1 AS grp,
               col1,
               col2
        FROM   t1
        UNION ALL
        SELECT 2 AS grp,
               col1,
               col2
        FROM   t2) AS t
GROUP  BY col1,
          col2
ORDER  BY MIN(grp),
          col1

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