如何在单个mysql select语句中使用Multiple where子句？

Question

我有一个名为marksheet的表，我存储了学生的标记。我想从每个单元考试中取出20％的分数，每周一次的考试分数为30％，期末考试分数为50％。单元考试，每周考试和期末考试的所有名称都存储在一个列名Name_of_exam下。我可以用

SELECT regd, 
       Sum(mark_score)/sum(Full_mark)*20 as scored 
FROM marksheet 
WHERE Name_of_exam='First Unit Exam' 
   OR Name_of_exam='Second Unit Exam' 
   OR Name_of_exam='Third Unit Exam' 
GROUP BY regd;

但问题是我不知道如何能够取出学生得分的30％： Where (Name_of_exam='First Weekly Test or Name_of_exam='Second Weekly Test' or Name_of_exam='Third Weekly Test') 并且50％的分数得分为Where Name_of_exam='Final Exam'。在同一个select语句中我必须做多个where子句。请帮忙。我还附上了如何在excel表中进行计算。

Answer 1

您不能使用多个WHERE部分，但始终可以使用逻辑运算符来加入条件。

此WHERE会在您的treshholds下找到得分的结果：

SELECT regd, names, subjects, 
  sum( score ) * (CASE WHEN (Name_of_exam='First Weekly Test or Name_of_exam='Second Weekly Test' or Name_of_exam='Third Weekly Test') THEN 0.2 
       WHEN ( Name_of_exam='First Unit Exam' or Name_of_exam='Second Unit Exam' OR Name_of_exam='Third Unit Exam')  THEN 0.3
       WHEN ( Name_of_exam='Final Exam' ) THEN 0.5 )
  AS scored
FROM marksheet
GROUP BY regd, names, subjects, test_type

Answer 2

试试这个：

SELECT names, subjects, 
SUM(
  CASE 
  WHEN name_of_exam = 'First Weekly Test' THEN (score/full_marks)*20 
  WHEN name_of_exam = 'First Unit Exam' THEN (score/full_marks)*30 
  WHEN name_of_exam = 'Final Unit Exam' THEN (score/full_marks)*50 
  END) AS scored
FROM marksheet
GROUP BY names, subjects