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  SQL Server Tips by Burleson

Constraints

Constraints are table properties that enforce data integrity and play a major role in the referential integrity process. There are five constraints: CHECK constraints, PRIMARY KEY, FOREIGN KEY, NOT NULL, UNIQUE.
Constraints can be column-level or table-level. Column-level constraints are checked when the column they are bound to is inserted or updated. Table-level constraints are checked when any column, from the table they are bound to, is inserted or updated.

CHECK constraints

The best way to restrict data from a column to a predefined format or range is by having a CHECK constraint in the column. This will happen when creating the table for the first time with CREATE TABLE or it can be done later with ALTER TABLE. CHECK constraints are more efficient than rules because a constraint is a table property while a rule is an object bound to the table.

Column-level constraint

Example: add a CHECK constraint to the phone column in the authors table in the Pubs database. The CHECK constraint will allow only data in the form 123 456-7890 or UNKNOWN.


The above book excerpt is from:

Super SQL Server Systems
Turbocharge Database Performance with C++ External Procedures

ISBN: 0-9761573-2-2
Joseph Gama, P. J. Naughter

 http://www.rampant-books.com/book_2005_2_sql_server_external_procedures.htm  

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