php - MySQL 为 0 时不递减

Question

我有

UPDATE topic SET liked = liked - 1 WHERE id='$id'

当liked为0时不递减。

当 liked 为 0 时，我必须做什么才能减少字段 liked？

注意:不为0时有效。

编辑:liked 是 int。

Answer 1

你想要下限为零吗？

UPDATE topic SET liked = liked - 1 WHERE id='$id' AND liked > 0;

如果你想要负数，你需要 SIGNED datatype当然。但是，您需要了解限制以避免错误。参见 "Out-of-Range and Overflow Handling"

所以假设 liked 更改为 SIGNED TINYINT...

UPDATE topic SET liked = liked - 1 WHERE id='$id' AND liked > -128;