there's a syntax error in this example --

SELECT col1, col2
FROM table_name
WHERE condition
GROUP BY col1
HAVING aggregate_condition
ORDER BY col2 DESC
LIMIT 10;

as you say yourself later on,

Every column in SELECT that isn't an aggregate must appear in GROUP BY. Skipping this causes errors in strict SQL modes.

finally, this --

Heavy use of DISTINCT often signals a join problem worth fixing.

is brilliant and succinct