关系代数 - sql（常用运算符）

Question

我有这些关系方案：

Participate (FestivalName, Artist)
Attend(ID, FestivalName)
Prefer (ID, FestivalName)
Like (ID, Artist) 

我想列出只参加节日的人 不是他们的偏好，同时，在其中之一 节日，至少有两位他们喜欢的艺术家参加（所以，2 或 更多的）。最后，出示身份证和节日。

我想通过使用通用运算符来解决这个问题：

selection operator, projection operator, union operator, difference operator and cartesian product

这是一个例子：

ATTEND TABLE
147|HannaBalusa |
147|FestivalTOP |
147|BestFestival|

PREFER TABLE
147|FestivalTOP|

LIKE TABLE
147|PaulMackarney|
147|BobDeylan    |

PARTICIPATE TABLE
HannaBalusa |PaulMackarney |
HannaBalusa |BobMurley     |
FestivalTOP |BobDeylan     |
BestFestival|PaulMackarney |
BestFestival|BobDeylan     |

所以，我应该得到这个输出：

147|BestFestival

我可以通过使用 SQL 来解决这种情况，但是我在使用关系代数时遇到了麻烦。

可以帮助我吗？

Answer 1

执行您想要的操作的惯用 SQL 查询可能如下所示，但它使用的 SQL 功能比您列出的要多。

SELECT A.ID, A.FestivalName
FROM Attend A
JOIN Participate P 
ON A.FestivalName = P.FestivalName --Adds rows with artists for each attended festival
JOIN Like L 
ON L.ID = A.ID AND L.Artist = P.Artist --Leaves only rows with `Like`d artists
WHERE NOT EXISTS (
  SELECT 1 
  FROM Prefer Pr 
  WHERE Pr.ID = A.ID AND Pr.FestivalName = A.FestivalName
) --Removes people who have ever attended a prefered festival
GROUP BY A.ID, A.FestivalName --Allows to count liked artists per festival 
HAVING COUNT(L.Artist) >= 2 --Leaves only festivals with 2 or more liked artists 

要使用您描述的操作，它可能看起来像这样

SELECT DISTINCT S1.ID, S1.FestivalName
FROM (
  SELECT A.ID, A.FestivalName, P.Artist
  FROM Attend A
  CROSS JOIN Participate P  
  CROSS JOIN Like L 
  WHERE A.FestivalName = P.FestivalName --Rows with artists for each attended festival 
  AND L.ID = A.ID AND L.Artist = P.Artist --Leaves only rows with `Like`d artists
) S1
CROSS JOIN ( -- Copy of the first subquery
  SELECT A.ID, A.FestivalName, P.Artist
  FROM Attend A
  CROSS JOIN Participate P  
  CROSS JOIN Like L 
  WHERE A.FestivalName = P.FestivalName
  AND L.ID = A.ID AND L.Artist = P.Artist 
) S2
WHERE S1.ID = S2.ID 
AND S1.FestivalName = S2.FestivalName 
AND S1.Artist != S2.Artist --Removes festivals with only 1 liked artist

MINUS -- Remove all rows with people who ever attended prefered festivals

SELECT ID, FestivalName 
FROM ( --People who attended prefered festivals
  SELECT DISTINCT A.ID
  FROM Attend A
  CROSS JOIN Prefer P
  WHERE A.ID = P.ID AND A.FestivalName = P.FestivalName
) 
CROSS JOIN ( -- All existent festivals
  SELECT FestivalName
  FROM Attend
)