Find the average of two combined columns in sql

By definition, AVG(col1) = SUM(col1)/COUNT(*) and AVG(col2) = SUM(col2)/COUNT(*), therefore (SUM(col1)+SUM(col2))/COUNT(*) = AVG(col1) + AVG(col2).

Also, the commutativity of addition gives us (SUM(col1)+SUM(col2))/COUNT(*) = SUM(col1+col2)/COUNT(*) and hence AVG(col1+col2).

To use the avg function,

SELECT avg(col1 + col2)
FROM test
WHERE uid=5;

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