有这样一张表，记录着用户和得分
1、需要展示一个排行榜
SELECT user_id, MAX(score) AS max_score FROM score_record GROUP BY user_id
2、排行榜中只展示60分以上的
SELECT user_id, max(score) as max_score from score_record GROUP BY user_id having max_score > 60 ORDER BY max_score desc
3、如果只是单纯的去重，查看有多少用户有得分记录
SELECT COUNT(DISTINCT(user_id)) as user_count from score_record
3、如果只是单纯的去重，查看都是哪些用户？
SELECT DISTINCT(user_id) AS user_count FROM score_record
