子查詢的MySQL

Question

如果我有這樣的一個表：

yr   subject 
1960 Physics 
1960 Chemistry 
1961 Physics 
1962 Chemistry 

我希望所有這些年來當主題是物理學，而不是化學

如果我試試這個，它並沒有給我正確的結果。

select yr from table where subject = "Physics" and 
yr NOT IN (select yr from table where subject = "Chemistry") 

Answer 1

您的查詢的問題在於，您在返回兩次物理獎項時返回了兩次。添加DISTINCT關鍵詞給你剛剛年（根據question 3A on this site）

SELECT DISTINCT yr FROM nobel 
WHERE 
    subject = 'Physics' AND 
    yr NOT IN (SELECT yr FROM nobel WHERE subject = 'Chemistry'); 

Answer 2

試試這個：

select yr from table t1 where subject = 'Physics' 
where not exists 
(select yr from table t2 where subject = 'Chemistry' and t1.yr = t2.yr) 

Answer 3

這其中解決了您的鏈接指出了一個問題：

SELECT DISTINCT n1.yr 
FROM nobel n1 
WHERE n1.subject = 'Physics' 
AND n1.yr NOT IN (
    SELECT DISTINCT n2.yr 
    FROM nobel n2 
    WHERE n2.subject = 'Chemistry' 
) 

原因這一個作品是因爲它消除了重複的年份。由於價格可以由同一年和多個主題的多個人授予，因此您希望刪除重複項。您也可以跳過內部查詢的DISTINCT，這會跳過一個排序步驟。在生產中，您需要分析此查詢並創建適當的索引。

Answer 4

在sqlzoo的問題是有點棘手，因爲沒有爲1933年等十餘一個物理學獎，正確的答案是：

select distinct yr from nobel as n1 
where subject = 'Physics' 
and not exists 
    (select 1 from nobel as n2 where n2.subject = 'Chemistry' and n2.yr = n1.yr) 

你可以利用這個查詢檢查結果1933年有：

select * from nobel where yr = 1933 

Answer 5

你幾乎沒有......你需要使用不同的：

select distinct yr from table where subject = "Physics" and 
yr NOT IN (select yr from table where subject = "Chemistry") 

對於鏈接給出：

select distinct yr from nobel 
where subject = 'Physics' and 
yr not in (select yr from nobel where subject = 'Chemistry')