SQL - 触发器

• 触发器是在插入、更新和删除语句前后自动执行的一堆SQL代码，但是触发器被触发后只会执行一次，通常我们使用触发器增强数据的一致性。

• 创建触发器
  *

  -- 创建触发器
  drop trigger if exists payments_after_insert;
  delimiter $$
  -- 在 payments表 insert 之后 触发 
  create trigger payments_after_insert
  	after insert on payments
      for each row 	-- 作用于受影响的每一行
  begin
  	update invoices
      set payment_total=payment_total + new.amount
      where invoice_id = new.invoice_id;
  end $$
  delimiter ;
  
  insert into payments
  values(default,5,3,'2019-01-01',10,1);
  select *
  from invoices;
  
  -- delete
  drop trigger if exists payments_after_delete;
  delimiter $$
  create trigger payments_after_delete
  	after delete on payments
      for each row
  begin
  	update invoices
      set payment_total=payment_total-old.amount
      where invoice_id=old.invoice_id;
  end$$
  delimiter ;
  
  delete 
  from payments
  where invoice_id=3;

• 查看触发器

  • show triggers like'payments%'

• 使用触发器进行审计

  • 触发器另一个常见用途，为了 审计的目的 而 记录对数据库的修改

  drop trigger if exists payments_after_insert;
  delimiter $$
  -- 在 payments表 insert 之后 触发 
  create trigger payments_after_insert
  	after insert on payments
      for each row 	-- 作用于受影响的每一行
  begin
  	update invoices
      set payment_total=payment_total + new.amount
      where invoice_id = new.invoice_id;
      
      insert into payments_audit		-- 审计记录
      values(new.client_id,new.date,new.amount,'insert',now());
  end $$
  delimiter ;
  
  
  drop trigger if exists payments_after_delete;
  delimiter $$
  create trigger payments_after_delete
  	after delete on payments
      for each row
  begin
  	update invoices
      set payment_total=payment_total-old.amount
      where invoice_id=old.invoice_id;
      
      insert into payments_audit		-- 审计记录
      values(old.client_id,old.date,old.amount,'delete',now());
  end$$
  delimiter ;
  
  
  insert into payments
  values(default,5,3,'2019-01-01',10,1);
  select *
  from invoices;
  
  delete 
  from payments
  where invoice_id=3;
  
  SELECT * FROM payments_audit;	-- 查询审计记录表