触发根据子表计数更新父列

Question

我有一张名为：

的表

Home

HomeID, HomeAddress, Capacity.

另一个叫：

Children

ChildID, HomeID,Childname.

有没有一种方法可以自动更新每个Home表容量列，其中包含在Children表中作为外键链接的子项数量？

Answer 1

你必须使用2个触发器才能实现。

1. 更新儿童后
2. 删除儿童后

示例1 ： 更新后 ：

delimiter //

drop trigger if exists au_on_children //

create trigger au_on_children after update on children 
for each row
begin
  declare old_totalCapacity int not null default 0;
  declare new_totalCapacity int not null default 0;

  select 
    case when homeID = OLD.homeID 
              then sum( OLD.homeID ) 
         else sum( homeID ) 
     end 
    into old_totalCapacity ,
    case when homeID = NEW.homeID 
              then sum( NEW.homeID ) 
         else sum( homeID )
     end 
    into new_totalCapacity 
    from children;

  update home 
     set capacity =  
         case when homeID = OLD.homeID 
                   then old_totalCapacity  
              else capacity 
         end ,
         case when homeID = NEW.homeID 
                   then new_totalCapacity 
              else capacity 
         end ;
end;
//

delimiter ;

示例1 ： 删除后 ：

delimiter //

drop trigger if exists ad_on_children //

create trigger ad_on_children after delete on children 
for each row
begin
  declare totalCapacity int not null default 0;

  select sum( homeID ) 
    into totalCapacity 
    from children 
   where homeID = OLD.homeID;

  update home 
     set capacity = totalCapacity 
   where homeId = OLD.homeID;
end;
//

delimiter ;