[sql] SQL: how to use UNION and order by a specific select?

Using @Adrian tips, I found a solution:

I'm using GROUP BY and COUNT. I tried to use DISTINCT with ORDER BY but I'm getting error message: "not a SELECTed expression"

select id from 
(
    SELECT id FROM a -- returns 1,4,2,3
    UNION ALL -- changed to ALL
    SELECT id FROM b -- returns 2,1
)
GROUP BY id ORDER BY count(id);

Thanks Adrian and this blog.

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