Create unique index across tables?

Question

Suppose I have the following

table a
serial_no varchar(8)  
serialised bit

table b:
serial_no varchar(8) (foreign key)
return_date datetime null

Can I create a unique index that does not allow duplicate records by serial_no and a null return_date, ONLY IF the serialised field from table a is true.

Something like:

create unique index unq_loan_serial_id
    on table b(serial_no) where return_date is null and table a.serialised = 1

Any ideas on how I can do this? Thanks

Answer 1

You cannot apply this directly to the table - but you could create an indexed view.

Something like:

CREATE VIEW dbo.b_serialised --Guessing everything in dbo schema
WITH SCHEMABINDING
AS
  SELECT
    b.Serial_No
  FROM
     dbo.a
       inner join
     dbo.b
       on
          a.Serial_No = b.Serial_No --Guessing this was the implied join
  WHERE
    b.return_date is null and
    a.serialised = 1

Followed by

create unique clustered index UQ_b_serialised on b_serialised (Serial_No)