Methods of Creating Indexes

You can create indexes on tables either by using the create index statement, or by using the unique or primary key integrity constraints of the create table command.

However, integrity constraints are limited in the following ways:

You cannot create nonunique indexes.
You cannot use the options provided by the create index command to tailor how indexes work.
You can only drop these indexes as a constraint using the alter table statement.

If your application requires these features, you should create your indexes using create index. Otherwise, the unique or primary key integrity constraints offer a simpler way to define an index for a table.