An index is a database object that orders the values of one or more columns in a table. An index provides pointers to the data values stored in specified columns of the table and orders the pointers in the specified order. When rows are requested from an indexed table, the database searches the index to find a particular value and then follows the pointer to the row containing that value.

Types of Data Integrity

SQL Server supports the following four types of data integrity:

1.Entity Integrity

Entity integrity defines a row as a unique entity for a particular table. Entity integrity enforces the integrity of the identifier column(s) or the primary key of a table (through indexes, UNIQUE constraints, PRIMARY KEY constraints, or IDENTITY properties).

2.Domain Integrity

Domain integrity validates the entries for a given column. Domain integrity can be enforced by restricting the type (through data types), the format (through CHECK constraints and rules), or the range of possible values (through FOREIGN KEY and CHECK constraints, DEFAULT definitions, NOT NULL definitions, and rules).

3.Referential Integrity

Referential integrity maintains the defined relationship between tables when records are entered or deleted from the tables. In SQL Server 2000, referential integrity is based on relationships between foreign keys and primary keys or between foreign keys and unique keys (through FOREIGN KEY and CHECK constraints). Referential integrity ensures that key values are consistent across the related tables.When referential integrity is enforced, SQL Server prevents users from adding records to a related table if there is no associated record in the primary table. Users are also prevented from changing values in a primary table or deleting records from the primary table if there are related records in the related table.

4.User-Defined Integrity

User-defined integrity is used to define specific business rules that do not fall into any of the other integrity categories. All of the integrity categories support user-defined integrity. All column-level and table-level constraints defined in CREATE TABLE, stored procedures, and triggers are examples of user-defined integrity.