sql - sql server 2008中的触发器

Question

我一直在研究，但似乎无法得到正确的结果。 我有下表:

create table school_tb
(idSchool int identity primary key,
nameSchool varchar(100),
schoolPopulation int
)

create table career_tb
(idCareer int identity primary key,
nameCareer       varchar(100),
carrerPopulation int,
numberClasses    int, 
idSchool int foreign key references school_tb(idSchool)
)

为了找出第一个表中的人口数量，我必须对同一学校的职业人口进行 SUM() 计算。 我需要创建一个触发器，当我更新 Career_tb 中的人口时，该触发器将更新表 school_tb 中的人口列。请帮我。 我有类似的东西，但我无法让它工作。

--create trigger updatePopulation
--on career_tb
--for update as
--if UPDATE(carrerPopulation)
--update school_tb set schoolPopulation =(SELECT add(carrerPopulation)
--                                  from career_tb
--                                  where idSchool=(SELECT idSchool
--                                  from career_tb
--                                  where idCareer=@idCareer)
--                                  )
--go

我很感激所提供的任何帮助。谢谢

Answer 1

这应该可以帮助你。请参阅触发器正文内的注释。

create trigger updatePopulation
on career_tb
-- to update sum even if carreer gets deleted or inserted
after insert, update, delete
as
-- to avoid trigger messing up rows affected
   set nocount on

   if UPDATE(carrerPopulation)
   begin
    -- update sum by difference between previous and current state of one record in career
      update school_tb
         set schoolPopulation = schoolPopulation + difference
        from school_tb
      -- derived table sums all the careers changed in one go
         inner join
         (
         -- sum all values from careers by school
             select idSchool, sum (carrerPopulation) difference
             from
             (
              -- change sign of previous values
                 select deleted.idSchool, -deleted.carrerPopulation carrerPopulation
                   from deleted
                 union all
              -- + current values
                 select inserted.idSchool, inserted.carrerPopulation
                   from inserted
              ) a
              group by idSchool
            -- Skip update in case of no change
              having sum (carrerPopulation) <> 0
         ) a
           on school_tb.idSchool = a.idSchool
    end