Question

我有两个桌子。

CREATE TABLE `departments` (
department_id INT(2) NOT NULL AUTO_INCREMENT,
department_name VARCHAR(30) NOT NULL,
total_employees INT(4),
PRIMARY KEY (department_id),
UNIQUE (department_name));

CREATE TABLE `employees` (
employee_id INT(4) NOT NULL AUTO_INCREMENT,
employee_email VARCHAR(30) NOT NULL,
employee_first_name VARCHAR(30) NOT NULL,
employee_last_name VARCHAR(30) NOT NULL,
department_name VARCHAR(30) NOT NULL,
PRIMARY KEY (employee_id),
UNIQUE (employee_email),
FOREIGN KEY (department_name)
    REFERENCES departments (department_name)
    ON DELETE CASCADE);

现在，假设我有一个人力资源管理部门。人力资源管理部门有2名员工。因此，我希望通过触发器来获取部门表2的total_employees的值。如果能得到您的一些建议，那就太好了。

Answer 1

您似乎想要一个 update 触发器，而不是一个 insert 触发器。插入触发器将是：

delimiter $$ 

create trigger employee_insertion_in_individual_department
after insert on employees for each row
begin
    update departments set 
        total_employees = total_employees + 1 
        where department_id = new.department_id;
    end if;
end$$

delimiter ;

更新触发器将更像：

delimiter $$ 
create trigger employee_insertion_in_individual_department
after insert on employees for each row
begin
    if not (old.employee_department <=> old.employee_department) then
        update departments set 
            total_employees = total_employees + (case when department_id = old_department_id then -1 else 1 end)
            where department_id in (old.department_id, new.department_id);
    end if;
end$$

delimiter ;

Answer 2

在插入触发器中，没有“旧” ...因为您要插入一行，而不是更新一行。该行没有以前的版本，因为它以前从未存在过。您可以只使用new在新创建的行中使用department_id。而且您不需要if，因为没有什么可比较：

delimiter $$ 
create trigger employee_insertion_in_individual_department
  after insert on employees for each row begin

    update departments set total_employees=total_employees+1 
    where department_id=new.department_id; 

end$$ delimiter ;

另请参阅How to get inserted value for trigger

P.S。通过使用实际行数而不是简单的增量来更新它可能更安全。我担心如果几乎同时更改许多行可能会产生歪斜。可能值得测试这是否引起问题。

P.P.S。我假设您还打算出于相同的目的添加UPDATE和DELETE触发器（如果还没有的话）。

如何在将值插入到另一个表的属性的基础上增加一个表的属性的值？

2 个答案: