SQL select，3表

Question

如何使用select如果我有3個表？

表：

school_subject(ID_of_subject, workplace, name) 
student(ID_of_student, firstName, surname, adress) 
writing(ID_of_action, ID_of_sbuject, ID_of_student) 

我想寫學生的名字和姓氏（按字母順序）誰擁有workplace=home。

這可能嗎？以及如何按字母順序？

SELECT s.firstName, s.surname 
FROM student S, school_subject P, writing Z 
WHERE P.workplace = 'home' 
     AND P.ID_of_subject = Z.ID_of_subject 
     AND Z.ID_of_student = s.ID_of_student; 

Answer 1

SELECT s.firstName, s.surname 
FROM student S INNER JOIN writing Z 
ON Z.ID_of_student = s.ID_of_student 
INNER JOIN school_subject P 
ON P.ID_of_subject = Z.ID_of_subject 
WHERE P.workplace = 'home' 
ORDER BY S.firstName, S.surname // Sort the list 

Answer 2

要按照字母順序排序的結果，可以使用ORDER BY關鍵字。因此，您的查詢就會變成：

SELECT DISTINCT S.firstName, S.surname 
FROM student S, school_subject P, writing Z 
WHERE P.workplace = 'home' AND 
P.ID_of_subject = Z.ID_of_subject AND 
Z.ID_of_student = S.ID_of_student 
ORDER BY S.surname, S.firstName 

的DISTINCT關鍵字是必要的，因爲在寫字檯上有給定鍵ID_of_subject和ID_of_student最終更多的元組。
所以這是必要的，以避免重複名字和姓氏多次。

Answer 3

注意，每個學生由ID_of_student確定，而不是由firstName和surname，從而@danjok說使用DISTINCT如果你只希望名字和姓氏。

如果要選擇滿足上SELECT條款您的要求（即使兩個或更多的學生有相同的firstName和surname），你應該包括ID_of_student所有學生：

SELECT S.ID_of_student, S.firstName, S.surname 
FROM student S 
INNER JOIN writing W ON W.ID_of_student = S.ID_of_student 
INNER JOIN school_subject P ON P.ID_of_subject = W.ID_of_subject 
WHERE P.workplace = 'home' 
ORDER BY S.firstName asc, S.surname asc