Mysql sql查询以获取参与的不同类的计数

我有一张游泳考勤表

 student_name  date_attended cnt
 Bob             21-Aug-2013  1 
 Bob             10-Sep-2013  1 

 student_name  date_attended cnt
 Bob             20-Aug-2013  1
 Bob             21-Aug-2013  1
 Bob             21-Aug-2013  1

我有一张空手道考勤表

 student_name  date_attended cnt
 Bob             21-Aug-2013  1 
 Bob             10-Sep-2013  1 

 student_name  date_attended cnt
 Bob             20-Aug-2013  1
 Bob             21-Aug-2013  1
 Bob             21-Aug-2013  1

我正在统计每个学生的游泳和空手道出勤人数

 student_name  swim_attendance  karate_attendance
 Bob               2              3

这里有一个方法

select
student_name,
coalesce(sum(swim_attendance)) as swim_attendance,
coalesce(sum(karate_attendance)) as karate_attendance
from
(
  ( 
     select student_name,sum(cnt) as swim_attendance, 0 as karate_attendance
     from swim_attendance group by student_name
  )
  union all
  ( 
     select student_name,0 as swim_attendance, sum(cnt) as karate_attendance
     from karate_attendance group by student_name
  )
)x
group by student_name
;

更新：从Straberry提出的点到sum（）仅一次

select
student_name,
coalesce(sum(swim_attendance)) as swim_attendance,
coalesce(sum(karate_attendance)) as karate_attendance
from
(
  ( 
     select student_name,cnt as swim_attendance, 0 as karate_attendance
     from swim_attendance 
  )
  union all
  ( 
     select student_name,0 as swim_attendance, cnt as karate_attendance
     from karate_attendance 
  )
)x
group by student_name
;

这里有一个方法

select
student_name,
coalesce(sum(swim_attendance)) as swim_attendance,
coalesce(sum(karate_attendance)) as karate_attendance
from
(
  ( 
     select student_name,sum(cnt) as swim_attendance, 0 as karate_attendance
     from swim_attendance group by student_name
  )
  union all
  ( 
     select student_name,0 as swim_attendance, sum(cnt) as karate_attendance
     from karate_attendance group by student_name
  )
)x
group by student_name
;

更新：从Straberry提出的点到sum（）仅一次

select
student_name,
coalesce(sum(swim_attendance)) as swim_attendance,
coalesce(sum(karate_attendance)) as karate_attendance
from
(
  ( 
     select student_name,cnt as swim_attendance, 0 as karate_attendance
     from swim_attendance 
  )
  union all
  ( 
     select student_name,0 as swim_attendance, cnt as karate_attendance
     from karate_attendance 
  )
)x
group by student_name
;

这里有一个方法

select
student_name,
coalesce(sum(swim_attendance)) as swim_attendance,
coalesce(sum(karate_attendance)) as karate_attendance
from
(
  ( 
     select student_name,sum(cnt) as swim_attendance, 0 as karate_attendance
     from swim_attendance group by student_name
  )
  union all
  ( 
     select student_name,0 as swim_attendance, sum(cnt) as karate_attendance
     from karate_attendance group by student_name
  )
)x
group by student_name
;

更新：从Straberry提出的点到sum（）仅一次

select
student_name,
coalesce(sum(swim_attendance)) as swim_attendance,
coalesce(sum(karate_attendance)) as karate_attendance
from
(
  ( 
     select student_name,cnt as swim_attendance, 0 as karate_attendance
     from swim_attendance 
  )
  union all
  ( 
     select student_name,0 as swim_attendance, cnt as karate_attendance
     from karate_attendance 
  )
)x
group by student_name
;

这里有一个方法

select
student_name,
coalesce(sum(swim_attendance)) as swim_attendance,
coalesce(sum(karate_attendance)) as karate_attendance
from
(
  ( 
     select student_name,sum(cnt) as swim_attendance, 0 as karate_attendance
     from swim_attendance group by student_name
  )
  union all
  ( 
     select student_name,0 as swim_attendance, sum(cnt) as karate_attendance
     from karate_attendance group by student_name
  )
)x
group by student_name
;

更新：从Straberry提出的点到sum（）仅一次

select
student_name,
coalesce(sum(swim_attendance)) as swim_attendance,
coalesce(sum(karate_attendance)) as karate_attendance
from
(
  ( 
     select student_name,cnt as swim_attendance, 0 as karate_attendance
     from swim_attendance 
  )
  union all
  ( 
     select student_name,0 as swim_attendance, cnt as karate_attendance
     from karate_attendance 
  )
)x
group by student_name
;

你可以这样做

select s.student_name ,
sum(cnt * coalesce(s.table_type = 'swim') ) swim_attendance,
sum(cnt * coalesce(s.table_type = 'karate') ) karate_attendance
from (
select student_name , 'swim' as table_type,cnt from swim_attendance 
union all
select student_name ,'karate' as table_type,cnt from karate_attendance
) s
group by s.student_name

你可以这样做

select s.student_name ,
sum(cnt * coalesce(s.table_type = 'swim') ) swim_attendance,
sum(cnt * coalesce(s.table_type = 'karate') ) karate_attendance
from (
select student_name , 'swim' as table_type,cnt from swim_attendance 
union all
select student_name ,'karate' as table_type,cnt from karate_attendance
) s
group by s.student_name

你可以这样做

select s.student_name ,
sum(cnt * coalesce(s.table_type = 'swim') ) swim_attendance,
sum(cnt * coalesce(s.table_type = 'karate') ) karate_attendance
from (
select student_name , 'swim' as table_type,cnt from swim_attendance 
union all
select student_name ,'karate' as table_type,cnt from karate_attendance
) s
group by s.student_name

你可以这样做

select s.student_name ,
sum(cnt * coalesce(s.table_type = 'swim') ) swim_attendance,
sum(cnt * coalesce(s.table_type = 'karate') ) karate_attendance
from (
select student_name , 'swim' as table_type,cnt from swim_attendance 
union all
select student_name ,'karate' as table_type,cnt from karate_attendance
) s
group by s.student_name

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没有两张桌子。你试过什么吗？我可以马上发布一个查询，但我想知道你的努力。为什么有一个

cnt

列？…另外，你没有主键。这是件坏事。没有两张桌子。你试过什么吗？我可以马上发布一个查询，但我想知道你的努力。为什么有一个

cnt

列？…另外，你没有主键。这是件坏事。没有两张桌子。你试过什么吗？我可以马上发布一个查询，但我想知道你的努力。为什么有一个

cnt

列？…另外，你没有主键。这是件坏事。没有两张桌子。你试过什么吗？我可以马上发布一个查询，但我想知道你的努力。为什么有一个

cnt

列？…另外，你没有主键。这是一件坏事。我想只有在我明白你的意思后，求和才会更有效率！！我想只有当我明白你的意思时，求和才会更有效率！！我想只有当我明白你的意思时，求和才会更有效率！！我想只有当我明白你的意思时，求和才会更有效率！！