Question

嗨，我有以下代码，我在其中尝试获取飞机ID（aircraftid）以及其服务的次数（count（serviceid）），但仅在count（serviceid）大于平均计数（服务ID）。

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

但是当我运行它时，它不返回任何行

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

Answer 1

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
)

Answer 2

您需要对avg子查询进行交叉联接

  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

或在haveing子句中使用子查询，因为您需要在select中选择aircraftid列，但不在..组中，而您需要总体平均值。

Answer 3

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;

Window函数是MySQL 8中引入的，是解决此问题的最佳方法。

获取子查询SQL的平均值

3 个答案: