本文介绍了将 Null 与 MySQL 触发器中的另一个值进行比较的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
So here is my issue I am comparing new and old values when a table row is being updated. But the new or old value will sometimes be null. So the code below doesn't work. Can can I remedy this issue?
Thank You
BEFORE UPDATE ON mytable
FOR EACH ROW
BEGIN
IF OLD.assignedto != NEW.assignedto
THEN
INSERT INTO history
(
asset ,
changedfield ,
oldvalue ,
newvalue
)
VALUES
(
NEW.asset,
'assignedto',
OLD.assignedto,
NEW.assignedto
);
END IF;
END$$
解决方案
Try:
IF ( (OLD.assignedto IS NOT NULL AND NEW.assignedto IS NOT NULL
AND OLD.assignedto != NEW.assignedto)
OR (OLD.assignedto IS NULL AND NEW.assignedto IS NOT NULL)
OR (OLD.assignedto IS NOT NULL AND NEW.assignedto IS NULL)
)
The comment is very much correct.
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