All nonaggregated columns in the SELECT clause must also be listed in the GROUP BY clause
If we're nitpicking
Even though I got it correct I don't think it is completely correct.
"extra" = 'X' is an expression not a column in the query and therefore is not bound by the above rule.
I saw solutions that used Group By without aggregates
SELECT col1 FROM [table] GROUP BY col1
yields the same result as
SELECT DISTINCT col1 FROM [table]
However GROUP BY includes an implicit COUNT(*) if not explicitly defined and therefore technically all GROUP BY's have an aggregate
Far away is close at hand in the images of elsewhere.