防止值在MySQL中变为负数的最佳方法 [英] Best way to prevent a value from going negative in mysql

问题描述

我们有一个表，该表通过在该表中记录交易来维护帐户余额.即最近的一行是帐户余额.

We have a table that maintains account balances by recording transactions in that table. i.e. the most recent row is the account balance.

记录提款时，我们要确保余额永远不会为负.我们建议的解决方案是这样的:

When recording a withdrawal, we would like to ensure that the balance can never go negative. Our proposed solution is something like:

INSERT INTO `txns`
  (`account_id`, `prev_balance`, `txn_type`, `new_balance`,  `amount`, `description`)
SELECT 
  t.account_id, t.new_balance, $txn_type, t.new_balance - $amount, $amount, $description
FROM`txns` t
WHERE t.account_id = '$account'
  AND (select new_balance 
        FROM txns 
        WHERE account_id = '$account' 
        ORDER BY txn_id desc limit 1) >= $amount
ORDER BY txn_id desc LIMIT 1;"

但是，如果使用ANDed子查询(我们在先前的项目中遇到子查询性能问题)，我们会有点担心性能.这里的开发人员都不是sql专家.存款没有附加条款.

But we are a bit concerned about the performance if the ANDed subquery (we had subquery performance issues on a previous project). None of the developers here are sql specialists. Deposits do not have the additional clause.

这一切都在MySQL 5.0上

This is all on MySQL 5.0

推荐答案

抱歉，我无法说出有关查询性能的任何信息.但是您可能希望考虑使用触发器来防止'new_balance'的情况变为负面. (因为在'new_balance'低于$ amount的情况下执行null插入使我感到很奇怪，但这仍然可行:)).

I cannot say anything about the performance of the query, sorry. But you might want to consider triggers to prevent the case of the 'new_balance' ever becoming negative. (Because it strikes me as odd to do a null-insert in case the 'new_balance' is lower than $amount, but it might work nevertheless :) ).

有关详细信息，请参见 MySQL 5.0的说明如何创建触发器.

See documenation of MySQL 5.0 for details how to create a trigger.

如果NEW.new_balance是负数，则基本上将其放入BEFORE触发器中.如果是，那么您将使用"STOP ACTION"(执行中的故意错误)中止触发器和INSERT查询.在评论中查看提及页面上的想法.

Basically you would put the check, if NEW.new_balance ís negative into a BEFORE-trigger. If yes, then you would use a "STOP ACTION", a deliberate error in execution, to abort the trigger and INSERT-query. See ideas on the mentioned page in the comments.

更新:修改了一下(我在家里安装MySQL的借口).

我的版本存在一个问题，即对于输入到moneylog中的每个值，都需要第二次写入数据库.

也许建议切换到存储过程.或者其他人有一个更好的主意，我对DB的兴趣不大:)

Update: Tinkered a little bit around (my excuse for installing MySQL at home).

My version has the problem of writing a second time to the DB for each value entered into moneylog.

Maybe switching to a stored proc would be advisable. Or somebody else has a better idea, I'm not that much into DB :)

CREATE DATABASE triggertest;
CONNECT triggertest;

CREATE TABLE transferlog (
  account SMALLINT UNSIGNED NOT NULL ,
  amount INT NOT NULL,
  new_balance INT NOT NULL
) ENGINE=INNODB;

CREATE TABLE stopaction (
  entry CHAR(20) NOT NULL,
  dummy SMALLINT,
  UNIQUE(`entry`)
);

INSERT INTO stopaction (`entry`) VALUES ('stop');

DELIMITER #
CREATE TRIGGER nonneg_insert BEFORE INSERT ON transferlog
  FOR EACH ROW BEGIN
    INSERT INTO stopaction (`entry`)
      SELECT CASE WHEN NEW.new_balance<0 THEN 'stop'
                  ELSE 'none' END;
    DELETE FROM stopaction WHERE entry!='stop';
  END;
#

CREATE TRIGGER nonneg_update BEFORE UPDATE ON transferlog
  FOR EACH ROW BEGIN
    INSERT INTO stopaction (`entry`)
      SELECT CASE WHEN NEW.new_balance<0 THEN 'stop'
                  ELSE 'none' END;
    DELETE FROM stopaction WHERE entry!='stop';
  END;
#
DELIMITER ;


INSERT INTO transferlog (`account`, `amount`, `new_balance`)  
  VALUES (1, 1000, 1000);
INSERT INTO transferlog (`account`, `amount`, `new_balance`)
  VALUES (1, -1000, 0);
INSERT INTO transferlog (`account`, `amount`, `new_balance`)
  VALUES (1, -1000, -1000);
INSERT INTO transferlog (`account`, `amount`, `new_balance`)
  VALUES (1, 10, 20);
SELECT version();

DROP DATABASE triggertest;

也许适合您，我对INSERT-Lines的输出是:

Maybe it will suit you, my output for the INSERT-Lines is:

mysql> INSERT INTO transferlog (`account`, `amount`, `new_balance`)  VALUES (1, 1000, 1000);
Query OK, 1 row affected (0.03 sec)

mysql> INSERT INTO transferlog (`account`, `amount`, `new_balance`)  VALUES (1, -1000, 0);
Query OK, 1 row affected (0.02 sec)

mysql> INSERT INTO transferlog (`account`, `amount`, `new_balance`)  VALUES (1, -1000, -1000);
ERROR 1062 (23000): Duplicate entry 'stop' for key 1

mysql> INSERT INTO transferlog (`account`, `amount`, `new_balance`)  VALUES (1, 10, 20);
Query OK, 1 row affected (0.02 sec)

mysql> SELECT version();
+---------------------+
| version()           |
+---------------------+
| 5.0.67-community-nt |
+---------------------+
1 row in set (0.00 sec)

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