SQL查询优化

假设您有以下表格：

TABLE Car
car_id
num

TABLE Car_description
dsc_id
car_id
model

TABLE Result
res_id
car_id
race_num
result

我们的任务是为超过2辆车的比赛选择最大平均分数，并为最佳汽车选择Car.num、Car\u description.model。解决此问题的一种方法是编写以下查询

select r.race_num,
       MAX(score) as MaxScore,
       AVG(score) as AvgScore,
       MAX(case when seqnum = 1 then c.carnum end) as TopCarNum,
       MAX(case when seqnum = 1 then cd.model end) as TopCarModel
from (select r.*,
             ROW_NUMBER() over (partition by race_num order by score desc) as seqnum
      from Result r
     ) r left outer join
     Car c
     on c.car_id = r.car_id left outer join
     Car_Description cd
     on c.car_id = d.car_id
group by r.race_num
having COUNT(*) > 2

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但在这里，我们在内部查询中对结果表进行排序，而不是选择最大元素，我认为这将非常耗时。是这样吗？我们如何优化这一点？谢谢。

我不确定这是否有助于提高性能，但请尝试

SELECT r.race_num,
   MAX(score) as MaxScore,
   AVG(score) as AvgScore,
   MAX(case when mr.max_score IS NOT NULL then c.carnum end) as TopCarNum,
   MAX(case when mr.max_score IS NOT NULL then cd.model end) as TopCarModel
FROM Result LEFT OUTER JOIN 
    (SELECT r.race_num, MAX(score) max_score FROM Result r GROUP BY r.race_num) mr 
    ON Result.res_num = mr.res_num AND Result.score = mr.max_score
 LEFT OUTER JOIN Car c
 ON c.car_id = Result.car_id 
 LEFT OUTER JOIN Car_Description cd
 ON c.car_id = d.car_id
 GROUP BY r.race_num
 HAVING COUNT(*) > 2

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“我想这会很费时”然后测试它。没关系，你只会忘记内部选择中的group by。谢谢