Best way to prevent a value from going negative in mysql

Question

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

When recording a withdrawal, we

Answer 1

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 :) ).

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

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.

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;

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)