如何在MySQL中强制执行唯一约束?

本文介绍了如何在MySQL中强制执行唯一约束?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

我定义了一个MySQL表:

I have a MySQL table defined:

File -------------------------- ID int(11) PK name varchar(100) customerId int(11) FK isPrimaryImage tinyint(1) ....

我在这里阅读到MySQL不支持带有WHERE条件的UNIQUE CONSTRAINT.对于每个 customerId ，我们只有一个 primaryImage = 1 .

I've read here that MySQL doesn't support a UNIQUE CONSTRAINT with a WHERE condition. For each customerId we'd have only one primaryImage = 1.

那么我还能采取什么措施来强制执行此约束?

So what else can I do to enforce this constraint?

推荐答案

MySQL完美支持唯一约束.

但是，它不支持部分约束/索引，因此您需要使用NULL而不是0标记非主图像.

It does not support partial constraints/indexes, though, so you would need to mark non-primary images with a NULL instead of 0.

ALTER TABLE file ADD CONSTRAINT ux_file_customer_primary UNIQUE (customerId, isPrimaryImage)

您可以在isPrimaryImage中插入任意数量的NULL值，但每个客户只能插入一个非空值.

You can insert arbitrary number of NULL values into isPrimaryImage but only one non-null value per customer.