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

Question

I have one table called marksheet where I stored students' marks. I want to take out 20% of mark scored out of each unit exam, 30% out of each weekly test and 50% out of final exam. All the names of Unit Exams, Weekly Tests and Final exam are stored under one column name Name_of_exam . I can use

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;

But the problem is that I have no idea how I can take out 30% of mark scored by students: Where (Name_of_exam='First Weekly Test or Name_of_exam='Second Weekly Test' or Name_of_exam='Third Weekly Test') and 50% of mark scored Where Name_of_exam='Final Exam' . in the same select statement as I have to do multiple where clauses. Please help. I have also attached here how I did the calculation in excel sheet.

Answer 1

You can't use multiple WHERE parts, but always you can use logical operators to join conditions.

This WHERE will find results with score under your treshholds:

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

Try this:

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