Question: Calculate the percentage of medium tip length in the list. List out the time when users of medium tip length join Yelp.
SQL: 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