I was testing some apparently wrong code and was quite sure it should return a syntax error. But it didn't. The following query works without errors (albeit doesn't sort the table either, which at least meets my expectations):
SELECT * FROM dummy ORDER BY 'id' 'desc';
Interestingly, that
SELECT * FROM dummy ORDER BY id 'desc';
does produce a syntax error.
How does MySQL interpret two strings after ORDER BY? What does it take these strings for? Here is the MCVE
SELECT * FROM dummy ORDER BY 'id' 'desc';
evaluates to
SELECT * FROM dummy ORDER BY 'iddesc';
I.e. ORDER BY a (constant) string literal - which doesn't affect the ORDER BY at all.
Note: This is ANSI/ISO SQL standard, a character literal can be built up by several parts, without having explicit concatenation.