SQL面試題---查詢最長連續登錄天數

給定Log表，查詢各用戶最長連續登錄天數。

     `Log`   
     +----------------+---------+     
     | user_id        | int     |     
     | login_time     | date    |       
     +----------------+---------+

 

解題思路：1，給各用戶的登錄日期組內排序。用窗口函數ROW_NUMBER實現。

                  2，因為row_number是連續的，所以如果login_time-row_number是恆定的，說明用戶從該login_time算起是連續登錄的。


                  3，計算各用戶login_time-row_number這個差值的數量，也就是各用戶的連續登錄天數。

                  4，按用戶id聚合，找出各用戶最大的連續登錄天數。

 

 答案：

WITH day_rank AS (
    SELECT user_id, 
           login_time, 
           ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY login_time) AS rank
    FROM Log),

day_diff AS (
    SELECT user_id, 
           DATE_SUB(login_time, rank) AS diff 
    FROM day_rank),

continuous_day AS (
    SELECT user_id, 
           COUNT(*) AS cnt
    FROM day_diff
    GROUP BY user_id, diff);

SELECT user_id, MAX(cnt) 
FROM continuous_day
GROUP BY user_id;