SQL加入澄清

Question

我希望爲醫院顯示hospitalid,hosp name and hosp type，這些醫院與醫生有關聯度最高。

我有兩個表：

醫生：doctorid, hospitalid

醫院：hospitalid, hname, htype

SELECT d.hospitalid,h.hname,h.htype 
FROM doctor d 
INNER JOIN hospital h ON d.hospitalid = h.hospitalid 
GROUP BY d.hospitalid,h.hname,h.htype 
HAVING MAX(count(d.doctorid)); 

我嘗試了上面的代碼，但我得到一個錯誤 「組func被嵌套太深」。我應該如何修改d代碼？

Answer 1

這是學習SQL時常見的錯誤，認爲having Max(col)說「只保留最大的行」。它只是意味着having <some function on the column>沒有任何條件。例如，你可以說having count(d.doctorid) = 1讓醫院只有一名醫生。

執行此操作的方法是對列進行排序然後取第一行。但是，「取第一行」的語法因數據庫而異。在許多SQL方言了以下工作：

SELECT d.hospitalid,h.hname,h.htype 
FROM doctor d INNER JOIN 
    hospital h 
    ON d.hospitalid = h.hospitalid 
GROUP BY d.hospitalid,h.hname,h.htype 
order by count(d.doctorid) desc 
limit 1; 

在SQL Server和Sybase，語法是：

SELECT top 1 d.hospitalid,h.hname,h.htype 
FROM doctor d INNER JOIN 
    hospital h 
    ON d.hospitalid = h.hospitalid 
GROUP BY d.hospitalid,h.hname,h.htype 
order by count(d.doctorid) desc; 

在Oracle：

select t.* 
from (SELECT d.hospitalid,h.hname,h.htype 
     FROM doctor d INNER JOIN 
      hospital h 
      ON d.hospitalid = h.hospitalid 
    GROUP BY d.hospitalid,h.hname,h.htype 
    order by count(d.doctorid) desc 
    ) t 
where rownum = 1; 

EDIT（基於評論）：

要獲取最大的所有行，可以執行類似於原始查詢的操作。這只是更復雜。你可以計算出使用子查詢的最大數量，並做having子句中的比較：

SELECT d.hospitalid, h.hname, h.htype 
FROM doctor d INNER JOIN 
    hospital h 
    ON d.hospitalid = h.hospitalid join 
GROUP BY d.hospitalid,h.hname,h.htype 
having count(d.doctorid) = (select max(NumDoctors) 
          from (select hospitalid, count(*) as NumDoctors 
            from hospitalId 
            group by hospitalid 
           ) hd 
          ) 

作爲一個說明，也有其他的數據庫更容易機制。

Answer 2

這就是我如何爲SQL Server編寫它。根據您使用的數據庫後端，具體細節可能會有所不同。

SELECT TOP 1 a.hospitalid,a.hname,a.htype 
FROM 
(SELECT d.hospitalid,h.hname,h.htype, count(d.doctorid) as doctorcount FROM doctor d INNER JOIN hospital h ON d.hospitalid = h.hospitalid 
GROUP BY d.hospitalid,h.hname,h.htype) a 
ORDER BY doctorcount DESC;