T-SQL - CREATE UNIQUE INDEX Keyword

The T-SQL (Transact-SQL) CREATE UNIQUE INDEX keyword is used to create indexes on a table with unique name. Indexes are useful to retrieve data from the database more quickly and speeds up the search queries. Indexes are not visible to the end users. Along with this, updating a table with indexes takes more time as compared to a table without any indexes because indexes are updated. Therefore it is advised to create indexes only on frequently searched columns.

The INSERT and UPDATE statements take more time on tables having indexes, whereas the SELECT statements become fast on those tables. The reason is that while performing insert or update operation, the database update the indexes as well.

T-SQL CREATE UNIQUE INDEX

Along with regular index, it is possible to create a unique index also on a table. Unique index is the same as the Primary key in SQL. The unique index indicates that the combination of values in the indexed columns must be unique. Unique indexes are used for the maintenance of the integrity of the data present in the table as well as for the fast performance.

Syntax

The syntax for using INDEX statement in T-SQL (Transact-SQL) is given below:

/* Allows only unique combination of 
   values in the indexed columns */
CREATE UNIQUE INDEX index_name
ON table_name (column1, column2, ...);

T-SQL CREATE UNIQUE INDEX Example

The below mentioned statement creates unique index on column Name of Employee table.

CREATE UNIQUE INDEX idx_name
ON Employee (Name);

To create a unique index on multiple columns (Name and EmpID) of a table (Employee), the statement is given below:

CREATE UNIQUE INDEX idx_empid
ON Employee (Name, EmpID);