Constraints provide a powerful yet easy way to enforce the data integrity in your database. Data integrity comes in three forms:
Entity integrity. Ensures that a table has a primary key. In SQL Server 2005, you can guarantee entity integrity by defining PRIMARY KEY or UNIQUE constraints or by building unique indexes. Alternatively, you can write a trigger to enforce entity integrity, but this is usually far less efficient.
Domain integrity. Ensures that data values meet certain criteria. In SQL Server 2005, domain integrity can be guaranteed in several ways. Choosing appropriate data types can ensure that a data value meets certain conditions–for example, that the data represents a valid date. Other approaches include defining CHECK ...