Mysql 获取子查询SQL的平均值

您好，我有下面的代码，我试图获取飞机ID（aircraftid）和它们被服务的次数（计数（serviceid）），但仅当计数（serviceid）大于平均计数（serviceid）时才显示它们

但当我运行它时，它不会返回任何行

当我将代码更改为

select aircraftid, s.times
from(
select aircraftid, count(serviceid) as times
from service
group by aircraftid
) as s
having times > 2;

它返回行

您可以试试这个-

SELECT aircraftid, 
COUNT(serviceid) AS times
FROM service
GROUP BY aircraftid
HAVING COUNT(serviceid) > (
    SELECT AVG(T) FROM 
    (
        SELECT count(serviceid) T
        FROM service
        GROUP BY aircraftid
    )A
)

---

在avg的子查询上需要交叉联接

  select aircraftid, s.times
  from(
    select aircraftid, count(serviceid) as times
    from service
    group by aircraftid
  ) as s
  cross join (
    select avg(times) avg_time
    from (
      select aircraftid, count(serviceid) as times
      from service
      group by aircraftid
    ) t
  ) t2 
  where s.times > t2.avg_time 

---

或者在having子句中使用子查询，因为您需要select中的aircraftid列，但它不在group by..中，并且您需要总体平均值..

我建议使用窗口功能：

select aircraftid, s.times
from (select aircraftid, count(*) as times,
             avg(count(*)) over () as avg_times
      from service
      group by aircraftid
     ) s
where times > avg_times;

MySQL 8中引入了窗口函数，是解决此问题的最佳方法

select aircraftid, s.times
from (select aircraftid, count(*) as times,
             avg(count(*)) over () as avg_times
      from service
      group by aircraftid
     ) s
where times > avg_times;