在你的评论之后编辑这个。

SELECT * 
 FROM (SELECT TOP 3 COUNT(*) AS numberofflights, origin, dest
       FROM flights
       WHERE origin = 'EWR'
       GROUP BY origin, dest
       UNION
       SELECT TOP 3 COUNT(*) AS numberofflights, origin, dest
       FROM flights
       WHERE origin = 'LGA'
       GROUP BY origin, dest
       ) A
 ORDER BY 2,1 desc

有两个相同的查询通过 union . 因为 union 删除重复项，这相当于：

select count(*) as numberofflights, origin
from flights
where destination = 'EWR'
group by origin
order by numberofflights desc;

不 union 是必要的。你的版本只是增加了很多不必要的开销。

对于mysql：
将每个查询都用括号括起来：

(
  select count(flight) as numberOfFlights, origin, dest
  from flights
  where origin = 'EWR'
  group by origin, dest
  order by numberOfFlights desc limit 3
)
union all
(
  select count(flight) as numberOfFlights, origin, dest
  from flights
  where origin = 'LGA'
  group by origin, dest
  order by numberOfFlights desc limit 3
)

对于sql server：

select * from 
(
  select top 3 count(flight) as numberOfFlights, origin, dest
  from flights
  where origin = 'EWR'
  group by origin, dest
  order by numberOfFlights desc
) t1
union all
select * from  
(
  select top 3 count(flight) as numberOfFlights, origin, dest
  from flights
  where origin = 'LGA'
  group by origin, dest
  order by numberOfFlights desc
) t2