Users of Microsoft SQL Server 7.0 can benefit from the new graphical SQL Server Query Analyzer and Index Tuning Wizard. These tools remove the guesswork from index creation. Nevertheless, understanding some basic index design recommendations can be useful to developers new to RDBMS.
The system creates a unique index on a column or columns referenced as a primary key in a CREATE TABLE or ALTER TABLE statement. The index ensures that no duplicate values are entered in the column(s) that comprise the primary key. SQL Server does not require you to declare a primary key for every table, but it is considered good programming practice for RDBMS developers.
You can optionally create unique or nonunique indexes on tables. These indexes can improve the performance of queries against the table.
Unique indexes improve performance and ensure that the values in the specified columns are unique.
Clustered indexes physically order the table data on the indexed column(s). Because the table can be physically ordered in only one way, only one clustered index can be created per table. If you have only one index on a table, it should be a clustered index. PRIMARY KEY constraints create clustered indexes automatically if no clustered index already exists on the table and a nonclustered index is not specified when you create the PRIMARY KEY constraint.