內部聯接Sql查詢

Question

我有一個包含3個表的問題：

1.Review（作業ID）

2.Job（BusinessNumber）

3.業務（BusinessNumber）（外鍵）。

我需要檢索數據並創建具有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
/

到目前為止，我還沒有創建表，我只是想顯示四列。

Answer 1

UNION運算符從具有相同投影的兩個集合中產生一個結果集。 您需要的是四列的新投影。 因此，將兩個子查詢轉換為內聯視圖並使用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
/

交叉聯接為聯接集中的每個記錄組合生成一個記錄。 當數據產生一項最高評級的業務和一項最低評級的業務時，這將很好。 但是，如果一個或兩個位置都存在平局，它將生成笛卡爾乘積。 有很多解決方法，因此，如果這對您不利，請完善您的問題。