SQL來獲得成績

Question

我正在創建一個應用程序來存儲測試。數據庫架構簡化下面

id_student   id_teacher   grade second_test 

student_1    teacher_1   10 'Y' 
student_1    teacher_1   5  'N' 
student_2    teacher_1   8  'N' 
student_3    teacher_1   9  'N' 

我需要一個查詢來獲取每個學生考慮，如果該學生有第二次測試（這是一個檢索）等級返回必須在第二個測試的一個檔次（而不是第一個）。

因此，對於這個例子中，結果集應

student_1    teacher_1   10 'Y' 
student_2    teacher_1   8  'N' 
student_3    teacher_1   9  'N' 

我寫了一個SQL語句，但它不工作。因爲它不會得到student_2和student_3

SELECT 
    m.id_student, 
    m.grade 
FROM 
    mytable m 
WHERE 
    second_test = 'Y' 
    OR EXISTS (
    SELECT 1 
    FROM 
     mytable 
    WHERE 
     id_student = m.id_student 
     AND id_teacher <> m.id_teacher 
     AND second_test = 'Y' 
) 

Answer 1

如果一個學生只能在你的表的一個或兩個記錄，你可以離開了參加第一次測試第二次測試。

select 
    t1.id_student, 
    t1.id_teacher, 
    coalesce(t2.grade,t1.grade) 
from 
    (select id_student, id_teacher, grade from my_table where second_test = 'N') as t1 
left join 
    (select id_student, id_teacher, grade from my_table where second_test = 'Y') as t2 
on 
    t1.id_student = t2.id_student and t1.id_teacher = t2.id_teacher 

如果一個學生可以有兩個以上的測試記錄，比你應該使用聚合方法在這兩個子querys。

Answer 2

你在這裏。 首先，讓所有有兩個年級的學生參加第二次考試，然後與那些只有一年級的學生合併。

SELECT 
    a.id_student, a.id_teacher, b.grade, b.second_test 
FROM (
    SELECT 
     id_student, id_teacher 
    FROM 
     mytable 
    GROUP BY 1,2 
    HAVING COUNT(*) > 1 
    ) a 
    JOIN mytable b ON a.id_student = b.id_student AND a.id_teacher = b.id_teacher AND b.second_test = 'Y' 
UNION 
SELECT 
    a.id_student, a.id_teacher, b.grade, b.second_test 
FROM (
    SELECT 
     id_student, id_teacher 
    FROM 
     mytable 
    GROUP BY 1,2 
    HAVING COUNT(*) = 1 
    ) a 
    JOIN mytable b ON a.id_student = b.id_student AND a.id_teacher = b.id_teacher 

這個答案可以很容易地修改，以獲得最新的檔次，即使你將有5級爲每個學生（取決於標誌）。