在SQL Server中，如何在层次结构模型中强制使用唯一的孙名称？

Question

Model: merchants have one to many customers and customers have one to many accounts. Accounts have names. All three tables have unique IDs for each row.

Constraint: For a given merchant, the account names must be unique.

How do we enforce this constraint in a SQL Server database schema?

Here'some ideas we've considered:

• We could add MerchantId to the Account table and create a unique constraint, but it's a redundant column to maintain given that CustomerId is already there. We'd need to make sure the combination of MerchantId and CustomerId are themselves consistent, so we'd make the foreign key between Account and Customer include both columns, even though CustomerId is already a unique identifier.
• We could add a check constraint to the Account table and use a UDF to check the constraint rule. But then a Customer could conceivable be assigned to a different Merchant, and the check constraint on Account wouldn't be checked. So we'd have to add another constraint on the Customer table, which starts to seem like we're doing it wrong, especially as the real model gets more complex than described here.
• We could enforce the constraint via triggers, but this doesn't seem to improve upon the shortcomings with using check constraints.

Answer 1

Maybe the best idea of a solution is to create a view joining the 3 tables and create a unique index on that view. That would be an indexed view . When you index a view it gets persisted just like a regular table, but it's updated automagically by the database engine as part of regular DML commands.

There are lots of requirements and restrictions on what you can and cannot do, those are in the docs I linked to above, but I think you can get away with it.

The code would go like this:

CREATE VIEW dbo.MerchantAccounts
WITH SCHEMABINDING
AS
    SELECT m.MerchantKey, a.AccountKey, a.Name
    FROM dbo.Accounts a
    INNER JOIN dbo.Customers c
        ON a.CustomerKey = c.CustomerKey
    INNER JOIN dbo.Merchants m
        ON c.MerchantKey = m.MerchantKey;
GO

CREATE UNIQUE CLUSTERED INDEX IX_MerchantKey_AccountName
ON dbo.MerchantAccounts (MerchantKey, Name);
GO

I included 3 columns in the view but only 2 are part of the unique clustered index. So you must not have duplicated MarchantKey,AccountName to begin with, and after that the database engine will ensure that for you.

You don't need to change your table and your relationships as long as you don't violate the requirements.

You can include more columns than just the key columns in your indexes view, and that can help performance for some queries. That's up to you. Just be aware that the resultset of the view (the equivalent of SELECT * FROM dbo.MerchantAccounts ) will be persisted on your database and will take up space. So the more columns you add the bigger the view gets and the more expensive it gets to maintain it up to date.