如何在 MySQL 中强制执行唯一约束？
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How to enforce unique constraint in MySQL?
提问by kasdega
I have a MySQL table defined:
我定义了一个 MySQL 表：
File
--------------------------
ID int(11) PK
name varchar(100)
customerId int(11) FK
isPrimaryImage tinyint(1)
....
I've read here that MySQL doesn't support a UNIQUE CONSTRAINTwith a WHEREcondition. For each customerIdwe'd have only one primaryImage = 1.
我在这里读到 MySQL 不支持UNIQUE CONSTRAINT带WHERE条件的a 。对于每个customerId，我们只有一个primaryImage = 1。
So what else can I do to enforce this constraint?
那么我还能做些什么来强制执行这个约束呢？
回答by Quassnoi
MySQLperfectly supports unique constraints.
MySQL完美支持唯一约束。
It does not support partial constraints/indexes, though, so you would need to mark non-primary images with a NULLinstead of 0.
它不支持部分限制/索引，虽然，所以你需要用标记非主图像NULL代替0。
ALTER TABLE file ADD CONSTRAINT ux_file_customer_primary
UNIQUE (customerId, isPrimaryImage)
You can insert arbitrary number of NULLvalues into isPrimaryImagebut only one non-null value per customer.
您可以NULL向isPrimaryImage每个客户插入任意数量的值，但只能插入一个非空值。
