在同一张桌子上执行合并

Question

我想对同一张表执行合并以将两个不同的结果组合在一起。理想的结果是拥有6条记录，这些记录代表每个区域（在这种情况下为两个区域EWR，LGA）的访问量最高的前3个目的地。 我标记了SQL Server和MySQL，因为我希望查询可以在两个RDBMS上运行。

Answer 1

对于 MySql ：
将您的每个查询括在括号内：

\text{}

对于 Sql Server ：

(
  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
)

Answer 2

您有两个由union连接的相同查询。由于union删除重复项，因此等效于：

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

不需要union。您的版本只会增加不必要的开销。

Answer 3

在评论后进行编辑。

 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