avg(a.event_date is not null)

This expression is not valid on its own as it requires additional context. It appears to be a part of a larger SQL statement or query that is used to calculate the average number of event dates that are not null in a given dataset or table.

The syntax of this expression suggests that it is using the AVG function, which is used to calculate the average value of a set of data. The expression inside the AVG function, "a.event_date is not null", is a conditional statement that filters the data to only include records where the event_date column is not null.

Overall, this expression is used to calculate the average number of non-null event dates in a dataset or table.

相关问题

select avg(if(q2.device_id is not null,1,0)) as avg_ret from (select distinct device_id,date from question_practice_detail) q1 left join (select distinct device_id,date_sub(date,interval 1 day) as date from question_practice_detail) q2 on q1.device_id = q2.device_id and q1.date = q2.date; 优化这段代码

这段代码主要是计算某个时间段内设备的平均留存率。为了优化这段代码，可以考虑以下几个方面：

1. 对子查询进行优化：将子查询中的DISTINCT操作改为GROUP BY操作，同时添加索引以加速查询速度。
2. 对JOIN操作进行优化：使用INNER JOIN代替LEFT JOIN，因为LEFT JOIN会对右表进行全表扫描，而INNER JOIN只会扫描符合条件的记录。
3. 避免使用函数：IF函数会降低查询速度，可以使用CASE WHEN语句代替。

优化后的代码如下所示：

SELECT AVG(CASE WHEN q2.device_id IS NOT NULL THEN 1 ELSE 0 END) AS avg_ret
FROM (
  SELECT device_id, DATE(date) AS date
  FROM question_practice_detail
  GROUP BY device_id, DATE(date)
) q1
INNER JOIN (
  SELECT DISTINCT device_id, DATE(date - INTERVAL 1 DAY) AS date
  FROM question_practice_detail
) q2
ON q1.device_id = q2.device_id AND q1.date = q2.date;

需要注意的是，以上优化方案并不一定适用于所有情况，具体还需要根据实际情况进行评估。

SELECT st.s_name, sc.s_score, avg(sc.s_score) from student st left JOIN score sc on st.s_id = sc.s_id GROUP BY sc.s_score HAVING sc.s_score < "60" OR sc.s_score is null返回的不是平均分

非常抱歉，您是对的，我的回答有误。查询语句中的`AVG(sc.s_score)`是为了计算每个分数对应的平均分数，而不是整个结果集的平均分数。如果要查询整个结果集的平均分数，可以使用以下语句：

SELECT AVG(sc.s_score) FROM student st LEFT JOIN score sc ON st.s_id = sc.s_id WHERE sc.s_score IS NOT NULL;

这个语句的作用是从学生表和分数表中选择非空分数，使用学生表的s_id和分数表的s_id进行连接，并计算所有非空分数的平均值。希望这次能够正确回答您的问题，感谢您的指正。