内部联接Sql查询

Question

我有一个包含3个表的问题：

1.Review（作业ID）

2.Job（BusinessNumber）

3.业务（BusinessNumber）（外键）。

我需要检索数据并创建具有4列HighestBusiness ， HighestRating ， LowestBusiness ， Lowest Rating 。

select business.businessname AS HIGHESTRATEDBUSINESS, 
        max(review.reviewrating) as HIGHEST
from business 
    inner join job 
        on job.selectedbusinessabnnumber=business.abnnumber 
    inner join review 
        on review.jobid=job.jobid 
where review.reviewrating = (select max(reviewrating)
                             from review)  
GROUP BY businessname 
UNION 
select business.businessname AS LOWESTRATEDBUSINESS, 
     MIN(review.reviewrating) as HIGHEST
from business 
    inner join job 
        on job.selectedbusinessabnnumber=business.abnnumber 
    inner join review 
         on review.jobid=job.jobid 
where review.reviewrating = (select MIN(reviewrating)
                               from review)
GROUP BY businessname
/

到目前为止，我还没有创建表，我只是想显示四列。

Answer 1

UNION运算符从具有相同投影的两个集合中产生一个结果集。 您需要的是四列的新投影。 因此，将两个子查询转换为内联视图并使用CROSS JOIN：

select h.highestratedbusiness
       , h.highest
       , l.lowestratedbusiness
       , l.lowest
from
    (select business.businessname AS HIGHESTRATEDBUSINESS, 
            max(review.reviewrating) as HIGHEST
    from business 
        inner join job 
            on job.selectedbusinessabnnumber=business.abnnumber 
        inner join review 
            on review.jobid=job.jobid 
    where review.reviewrating = (select max(reviewrating)
                                from review)  
    GROUP BY businessname ) h
cross join 
        (select business.businessname AS LOWESTRATEDBUSINESS, 
             MIN(review.reviewrating) as HIGHEST
        from business 
            inner join job 
                on job.selectedbusinessabnnumber=business.abnnumber 
            inner join review 
                 on review.jobid=job.jobid 
        where review.reviewrating = (select MIN(reviewrating)
                                       from review)
        GROUP BY businessname ) l
/

交叉联接为联接集中的每个记录组合生成一个记录。 当数据产生一项最高评级的业务和一项最低评级的业务时，这将很好。 但是，如果一个或两个位置都存在平局，它将生成笛卡尔乘积。 有很多解决方法，因此，如果这对您不利，请完善您的问题。