用户在牛客试卷作答区作答记录存储在表exam_record中，内容如下： exam_record表（uid用户ID, exam_id试卷ID, start_time开始作答时间, submit_time交卷时间, score得分） id uid exam_id start_time submit_time score 1 1001 9001 2021-07-02 09:01:01 2021-07-02 09:21:01 80 2 1002 9001 2021-09-05 19:01:01 2021-09-05 19:40:01 81 3 1002 9002 2021-09-02 12:01:01 (NULL) (NULL) 4 1002 9003 2021-09-01 12:01:01 (NULL) (NULL) 5 1002 9001 2021-07-02 19:01:01 2021-07-02 19:30:01 82 6 1002 9002 2021-07-05 18:01:01 2021-07-05 18:59:02 90 7 1003 9002 2021-07-06 12:01:01 (NULL) (NULL) 8 1003 9003 2021-09-07 10:01:01 2021-09-07 10:31:01 86 9 1004 9003 2021-09-06 12:01:01 (NULL) (NULL) 10 1002 9003 2021-09-01 12:01:01 2021-09-01 12:31:01 81 11 1005 9001 2021-09-01 12:01:01 2021-09-01 12:31:01 88 12 1006 9002 2021-09-02 12:11:01 2021-09-02 12:31:01 89 13 1007 9002 2020-09-02 12:11:01 2020-09-02 12:31:01 89 请计算2021年每个月里试卷作答区用户平均月活跃天数avg_active_days和月度活跃人数mau，上面数据的示例输出如下： month avg_active_days mau 202107 1.50 2 202109 1.25 4 解释：2021年7月有2人活跃，共活跃了3天（1001活跃1天，1002活跃2天），平均活跃天数1.5；2021年9月有4人活跃，共活跃了5天，平均活跃天数1.25，结果保留2位小数。 注：此处活跃指有交卷行为。
示例1

输入

drop table if exists exam_record;
CREATE TABLE exam_record (
    id int PRIMARY KEY AUTO_INCREMENT COMMENT '自增ID',
    uid int NOT NULL COMMENT '用户ID',
    exam_id int NOT NULL COMMENT '试卷ID',
    start_time datetime NOT NULL COMMENT '开始时间',
    submit_time datetime COMMENT '提交时间',
    score tinyint COMMENT '得分'
)CHARACTER SET utf8 COLLATE utf8_general_ci;

INSERT INTO exam_record(uid,exam_id,start_time,submit_time,score) VALUES
(1001, 9001, '2021-07-02 09:01:01', '2021-07-02 09:21:01', 80),
(1002, 9001, '2021-09-05 19:01:01', '2021-09-05 19:40:01', 81),
(1002, 9002, '2021-09-02 12:01:01', null, null),
(1002, 9003, '2021-09-01 12:01:01', null, null),
(1002, 9001, '2021-07-02 19:01:01', '2021-07-02 19:30:01', 82),
(1002, 9002, '2021-07-05 18:01:01', '2021-07-05 18:59:02', 90),
(1003, 9002, '2021-07-06 12:01:01', null, null),
(1003, 9003, '2021-09-07 10:01:01', '2021-09-07 10:31:01', 86),
(1004, 9003, '2021-09-06 12:01:01', null, null),
(1002, 9003, '2021-09-01 12:01:01', '2021-09-01 12:31:01', 81),
(1005, 9001, '2021-09-01 12:01:01', '2021-09-01 12:31:01', 88),
(1006, 9002, '2021-09-02 12:11:01', '2021-09-02 12:31:01', 89),
(1007, 9002, '2020-09-02 12:11:01', '2020-09-02 12:31:01', 89);

输出

202107|1.50|2
202109|1.25|4
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