The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table. A UNION statement effectively does a SELECT DISTINCT on the results set.
The hottest layer of the Earth is its innermost layer. The inner core Quite literally the center of the Earth, the inner core is solid and can get to estimate temperatures over 9,000 degrees Fahrenheit