An index is a disk-based structure linked to a table or view that facilitates quicker row retrieval.  A table or view's table or view's columns are used to create keys in an index. These keys are kept in a structure (B-tree) that enables SQL Server to quickly locate the row or rows that correspond to the key values. The users cannot see the indexes, they are merely used to speed up queries. The CREATE INDEX statement is used to create indexes in tables.  Syntax:  CREATE INDEX index_name
ON table_name (column_1, column_2, ... , column_N); For indexing purposes, NULL values in a column are considered duplicate values if they are repeated more than once. Indexing is not possible if you have duplicate entries in a column. Therefore, creating a unique index in a column with NULL values is only possible if there is only one NULL value present in the respective field.