Like a Join a UNION merges data in the query result, But this is more like a Cut and Paste Operation with rules for getting a reasonable result
In General the 2 data results (Usually a table but could be a query result) being merged must have a complementary sequence of fields (Best if they are identical)
SELECT *
FROM planets
LEFT JOIN moons USING (Diameter_km)
UNION ALL
SELECT *
FROM planets
RIGHT JOIN moons USING (Diameter_Km)
ORDER BY Diameter_km;
We actually already did one in a previous example
These 2 queries being joined produce the exact same field layout
The union just pasted the 2nd result to the bottom of the first