How Can I Use a Custom Function to Enforce Data Integrity Between Tables in SQL Server 2008?

Using Custom Functions with Check Constraints in SQL Server 2008

In SQL Server 2008, managing data integrity between multiple tables can be challenging. This article addresses a specific issue in maintaining consistent data between the venues and events tables.

Problem Statement

A custom function is required to implement a check constraint that ensures that the event_expected_attendance in the events table never exceeds the venue_max_capacity in the venues table. However, the complexities of joining multiple tables and custom function syntax present a challenge.

Solution

While implementing a check constraint with a user-defined function (UDF) can impact performance, the following code provides a functional solution:

CREATE FUNCTION dbo.CheckVenueCapacity (@venue_id int, @capacity int)
RETURNS int
AS
BEGIN
  DECLARE @retval int
  SELECT @retval = CASE WHEN venue_max_capacity >= @capacity THEN 0 ELSE 1 END
  FROM venues
  WHERE venue_id = @venue_id
  RETURN @retval
END;
GO

ALTER TABLE events
ADD CONSTRAINT chkVenueCapacity
CHECK (dbo.CheckVenueCapacity(event_venue_id, event_expected_attendance) = 0);

This function, CheckVenueCapacity, takes the venue_id and the expected attendance as input parameters and returns 0 if the expected attendance is within the venue's maximum capacity, otherwise it returns 1. The check constraint chkVenueCapacity is then added to the events table to enforce this condition. By using a custom function in the check constraint, you can maintain the data integrity between the venues and events tables.

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