Working with Constraints (Visual Database Tools)
Constraints are rules that the database server enforces for you.
If the table is published for replication, you must make schema changes using the Transact-SQL statement ALTER TABLE or SQL Server Management Objects (SMO). When schema changes are made using the Table Designer or the Database Diagram Designer, it attempts to drop and recreate the table. You cannot drop published objects, therefore the schema change will fail.
For details about working with constraints, see the following topics.
Ensure no duplicate values are entered in specific columns
Change the columns that the constraint is attached to, change the constraint name, or set additional properties for the constraint
Remove the requirement for uniqueness for values entered in the column
Create a constraint expression to check data for a condition
Apply constraints either to new data only or to existing data as well
Attach a check constraint to a table to specify the data values that are acceptable in one or more columns
Disable a check constraint when a replication agent inserts or updates data in your table
Disable a check constraint when data is added to, updated in, or deleted from a table
Change the constraint expression or the options that enable or disable the constraint for specific conditions
Remove the limitations on data values in a column
Show the properties for a selected check constraint in the Properties window