Question

How could I set a constraint on a table so that only one of the records has its isDefault bit field set to 1?

The constraint is not table scope, but one default per set of rows, specified by a FormID.

Answer 1

Here’s a modification of Damien_The_Unbeliever’s solution that allows one default per FormID.

CREATE VIEW form_defaults AS SELECT FormID FROM whatever WHERE isDefault = 1 GO CREATE UNIQUE CLUSTERED INDEX ix_form_defaults on form_defaults (FormID) GO 

But the serious relational folks will tell you this information should just be in another table.

CREATE TABLE form FormID int NOT NULL PRIMARY KEY DefaultWhateverID int FOREIGN KEY REFERENCES Whatever(ID)