CHECK constraints are applied to rows generated as candidates for INSERT and UPDATE operations.
You cannot specify CHECK constraints for Identity columns.
If the condition is met or the proposed INSERT or UPDATE contains NULLs, the operation is permitted.
If the condition is not met, an error is reported as a constraint violation. For details on the syntax, see “CREATE TABLE” in SQL Data Definition Language.
For character data, constraints are checked using the current session collation. Therefore, it is possible for a value to meet the constraint for one collation and violate the constraint for another collation.
Also see “Constraint Specifications” in Database Design.