Question

我有一张像

这样的表格

Student Subject Marks
John    Maths   80
John    Science 70
John    Social  90
Raju    Maths   90
Raju    Science 80
Raju    Social  70

我希望所有受试者的平均数学和其他所有受试者的平均值如下所示，使用＆＃34;有＆＃34;子句。

我想要这样的O / P

Subject                   Average

Maths                       85

Rest(of all subjects)       77.5

Answer 1

使用CASE：

SELECT
   CASE
     WHEN Subject = 'Math' THEN Subject 
     ELSE 'All Subjects'
   END,
   AVG(Marks)
FROM tab
GROUP BY
   CASE
     WHEN Subject = 'Math' THEN Subject 
     ELSE 'All Subjects'
   END

Answer 2

case语句可以用作分组字段，如下所示：

select CASE subject WHEN 'Maths' THEN subject ELSE 'Others' END as Subject, 
       AVG(Marks) as Average
from tbl
group by CASE subject WHEN 'Maths' THEN subject ELSE 'Others' END

希望有所帮助。

Answer 3

使用CASE表达式在派生表中进行主题替换。然后对它进行GROUP BY的结果：

select subject, avg(marks)
from 
(
    select case when Subject <> 'Maths' then 'Rest(of all subj)' else subject end,
           Marks
    from tablename
) dt
group by subject

（核心ANSI SQL-99，加上F591，“派生表”功能。）

没有CASE：

select Subject, avg(marks)
from
(
select Subject, marks from tablename where Subject = 'Maths'
union all
select 'Rest(of all subj)', marks from tablename where Subject <> 'Maths'
)
group by Subject

或者甚至更简化，正如dnoeth所建议的那样：

select 'Maths' as Subject,  avg(marks) from tablename where Subject = 'Maths'
union all
select 'Rest(of all subj)', avg(marks) from tablename where Subject <> 'Maths'

Answer 4

条件聚合的另一种方式。当然，它可能会混淆一些。

AVG(1,2,NULL) = 1.5，不 1因为，聚合函数排除 NULL

SELECT
   AVG(CASE
     WHEN Subject = 'Maths' THEN Marks 
     ELSE NULL
   END) "Maths",
 AVG(CASE
     WHEN Subject = 'Maths' THEN NULL 
     ELSE Marks
   END)  "Rest Of All"
FROM tab

所有受试者的平均值除外

4 个答案: