Calculate the percentage of medium tip length in the list. List out the time when users of medium tip length join Yelp.
medium tip length refers to tip_length = 'Medium'; percentage = divide(count(tips where tip_length = 'Medium'), count(tips))*100%; the time when users join Yelp refers to user_yelping_since_year
SELECT CAST(SUM(CASE WHEN T1.tip_length = 'Medium' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T1.tip_length), T2.user_yelping_since_year FROM Tips AS T1 INNER JOIN Users AS T2 ON T1.user_id = T2.user_id