Question

是否有更好的方法可以在不使用嵌套子查询的情况下执行此查询？

-- select all races for jockeys which have multiple regions
select distinct r.id, r.description
from jockeys_races jra inner join races r on r.id = jra.race_id
where jra.jockey_id in (
    select jre.jockey_id
    from jockeys_regions jre
    group by jre.jockey_id
    having count(*) > 1
);

骑师可以属于多个地区。骑师可以参加多场比赛。

Answer 1

它可能效率不高（因为产品会更大），但你可以做到。检查两者上的EXPLAIN。

select distinct r.id, r.description
from races r
  inner join jockey_races jra on r.id = jra.race_id
  inner join jockey_regions jre on jra.jockey_id = jre.jockey_id
group by r.id, r.description, jre.jockey_id
having count(*) > 1

Answer 2

你可以这样做，我相信：

select distinct r.id, r.description
from jockeys_races jra 
inner join races r on r.id = jra.race_id
inner join jockeys_regions jre on jre.jockey_id=jra.jockey_id
group by r.id, r.description, jre.jockey_id
having count(jre.region) > 1

Answer 3

您不希望在MySQL中使用IN和子查询。。。优化器在这个构造中做得很差。在这种情况下，您可以将子查询移动到FROM子句：

select distinct r.id, r.description
from jockeys_races jra inner join
     races r
     on r.id = jra.race_id join
     (select jre.jockey_id
      from jockeys_regions jre
      group by jre.jockey_id
      having count(*) > 1
     ) ji
     on jra.jockey_id = ji.jockey_id

避免包含组的嵌套子查询

3 个答案: