When a row is inserted in a table with a default definition for a column, the SQL Server is implicitly instructed to insert the specified default value in the column if a value is not specified for the column.

IDENTITY Properties

The IDENTITY property is used to define a column as an identifier column. An identifier column contains system-generated sequential values that uniquely identify each row in the table. A table can have only one identifier column. Identifier columns usually contain values that are unique only within the table for which they have been defined. In other words, other tables containing identifier columns can contain the same identity values used by another table. However, the identifier values are typically used only within the context of a single table, and the identifier columns do not relate to other identifier columns in other tables.

Constraints

Constraints are used to define the way that SQL Server automatically enforces the integrity of a database. A constraint is a property assigned to a table or column within a table that prevents invalid data values from being entered in the specified column(s). For example, a PRIMARY KEY or UNIQUE constraint on a column prevents a duplicate value from being inserted into the column. A CHECK constraint on a column prevents the column from accepting a value that does not meet the specified condition. Moreover, a FOREIGN KEY constraint establishes a link between data in two tables.

Rules

Rules perform some of the same functions as CHECK constraints. However, CHECK constraints are preferred over rules. Rules are provided only for backward compatibility. CHECK constraints are more concise than rules. A column can have only one rule applied to it. However, multiple CHECK constraints can be applied to a column. CHECK constraints are specified while creating a table, whereas rules are created as separate objects and are bound to the column.

The CREATE RULE statement is used to create a rule. Once a rule has been created, it can be bound to a column or a user-defined data type by using the sp_bindrule system stored procedure.

Triggers

Triggers are special types of stored procedures that are defined to execute automatically when an UPDATE, INSERT, or DELETE statement is issued against a table or view. Triggers can be used to enforce business rules automatically when data is modified. Triggers can also be used to extend the integrity checking logic of constraints, defaults, and rules. However, it is recommended that constraints and defaults be used instead of triggers whenever they provide all of the needed functionality.

Indexes