Error validating constraint. (Visual Database Tools)
The text of the Constraint Expression property does not conform to the constraint syntax recognized by the DBMS. Some common reasons why a constraint expression can violate the required syntax are:
The expression does not resolve to a value of TRUE or FALSE.
The expression misspells a column name, or refers to a column in another table.
The expression misspells the name of a function or database constant.
For a complete description of the syntax for constraint expressions in SQL Server 2000, see the SQL Server documentation.