mysqli每次只工作else条件而不是if条件

Question

这是我的查询

CREATE VIEW marksheet as
SELECT name as name, student_id as student_id, 
roll as roll, class as class,exam_year as exam_year, 
subject_name as subject, exam_type as exam_type,
sum(full_mark) as full_mark, sum(getmark) as getmark,
department as department,
IF(SUM(IF(gpa='f' OR gpa='F',-9999,gpa))>=0, 
CAST(IF(subject_type=1,SUM(gpa)-2/count(subject_name),SUM(gpa)/count(subject_name)) 
AS CHAR), 'F') as total_gpa
FROM mark
GROUP by roll, class, exam_type

不工作

IF(subject_type=1,SUM(gpa)-2/count(subject_name),SUM(gpa)/count(subject_name))

每次只工作else condation SUM(gpa)/count(subject_name

不工作subject_type=1,SUM(gpa)-2/count(subject_name)

我的桌子

结果：gpa = 5+8+4+6

         = 23

但是 subject_type = 1 so ,minus -2（不起作用）

         = 21 (Not work)

最终 Gpa = 21/count(subject_name)

Answer 1

如果只能有一个额外的科目（即带有subject_type=1 的科目）并且所有其他科目都有subject_type=0，您可以使用MAX(subject_type) 来确定学生是否参加了额外的科目。这个查询应该做你想做的事（注意你还需要() 围绕SUM(gpa)-2）：

CREATE VIEW marksheet as
SELECT name as name, student_id as student_id, 
roll as roll, class as class,exam_year as exam_year, 
subject_name as subject, exam_type as exam_type,
sum(full_mark) as full_mark, sum(getmark) as getmark,
department as department,
IF(SUM(IF(gpa='f' OR gpa='F',-9999,gpa))>=0, 
CAST(IF(MAX(subject_type)=1,(SUM(gpa)-2)/count(subject_name),SUM(gpa)/count(subject_name)) 
AS CHAR), 'F') as total_gpa
FROM mark
GROUP by roll, class, exam_type

我在SQLFiddle 创建了一个简化的演示。