Question

假设我有以下数据库架构： schema

学生可以在同一科目中获得2个以上的成绩。

我想更新表学生的列highest_math_grade，highest_physic_grade，highest_chemistry_grade。我就是这样做的：

UPDATE student JOIN (
SELECT student.id, MAX(grade) AS grade
FROM student join grade
ON student.id = grade.student_id
WHERE subject = 'math'
GROUP BY student.id
  ) AS subquery
ON student.id = subquery.id
SET highest_math_grade = subquery.grade;

UPDATE student JOIN (
    SELECT student.id, MAX(grade) AS grade
    FROM student join grade
    ON student.id = grade.student_id
    WHERE subject = 'physic'
    GROUP BY student.id
  ) AS subquery
ON student.id = subquery.id
SET highest_physic_grade = subquery.grade;


UPDATE student JOIN (
    SELECT student.id, MAX(grade) AS grade
    FROM student join grade
    ON student.id = grade.student_id
    WHERE subject = 'chemistry'
    GROUP BY student.id
) AS subquery
ON student.id = subquery.id
SET highest_chemistry_grade = subquery.grade;

我不得不使用3个更新语句。我试过了：

UPDATE student
  JOIN (SELECT *
        FROM grade
        ORDER BY grade) AS grade
    ON student.id = grade.student_id
SET highest_math_grade    = (CASE WHEN grade.subject = 'math'
  THEN grade.grade
                             ELSE highest_math_grade END),
  highest_chemistry_grade = (CASE WHEN grade.subject = 'chemistry'
    THEN grade.grade
                             ELSE highest_chemistry_grade END),
  highest_physic_grade    = (CASE WHEN grade.subject = 'physic'
    THEN grade.grade
                             ELSE highest_physic_grade END);

然而，它只会更新每个学生一次，这不是我想要的。

有没有办法只用一个更新语句来实现同样的目的？

Answer 1

试试这个答案，希望这会对你有所帮助：

UPDATE S SET S.highest_math_grade=Math_Grade
    ,S.highest_chemistry_grade =chemistry_Grade
    ,S.highest_physic_grade=physic_Grade
FROM STUDENT S,(
    SELECT S.ID
        ,MAX(G1.grade)Math_Grade
        ,MAX(G2.grade)chemistry_Grade
        ,MAX(G3.grade)physic_Grade
    FROM STUDENT S
    LEFT JOIN Grade G1 ON S.id=G1.student_id AND G1.subject = 'math'
    LEFT JOIN Grade G2 ON S.id=G2.student_id AND G2.subject = 'chemistry'
    LEFT JOIN Grade G3 ON S.id=G3.student_id AND G3.subject = 'physic'
    GROUP BY S.ID
)D
WHERE S.ID=D.ID

更新同一语句中的多个列

1 个答案: