MySQL 完美支持唯一约束.

不过，它不支持部分约束/索引，因此您需要使用 NULL 而不是 0 来标记非主要图像.

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.