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 $$
    -- 在 payments表 insert 之后 触发 
    create trigger payments_after_insertafter insert on paymentsfor each row 	-- 作用于受影响的每一行
    beginupdate invoicesset payment_total=payment_total + new.amountwhere invoice_id = new.invoice_id;
    end $$
    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 $$
    create trigger payments_after_deleteafter delete on paymentsfor each row
    beginupdate invoicesset payment_total=payment_total-old.amountwhere invoice_id=old.invoice_id;
    end$$
    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 $$
    -- 在 payments表 insert 之后 触发 
    create trigger payments_after_insertafter insert on paymentsfor each row 	-- 作用于受影响的每一行
    beginupdate invoicesset payment_total=payment_total + new.amountwhere invoice_id = new.invoice_id;insert into payments_audit		-- 审计记录values(new.client_id,new.date,new.amount,'insert',now());
    end $$
    delimiter ;drop trigger if exists payments_after_delete;
    delimiter $$
    create trigger payments_after_deleteafter delete on paymentsfor each row
    beginupdate invoicesset payment_total=payment_total-old.amountwhere invoice_id=old.invoice_id;insert into payments_audit		-- 审计记录values(old.client_id,old.date,old.amount,'delete',now());
    end$$
    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