SQL Server - INDEX
The SQL Server (Transact-SQL) INDEX statement is used to create indexes on a table. 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.
SQL Server CREATE 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.
The syntax for using CREATE INDEX statement in SQL Server (Transact-SQL) is given below:
/* Allows duplicate combination of values in the indexed columns */ CREATE INDEX index_name ON table_name (column1, column2, ...); /* Allows only unique combination of values in the indexed columns */ CREATE UNIQUE INDEX index_name ON table_name (column1, column2, ...);
SQL Server CREATE INDEX Example
The below mentioned query creates index on column Name of Employee table.
CREATE INDEX idx_name ON Employee (Name);
To create a index on multiple columns (Name and EmpID) of a table (Employee), the query is given below:
CREATE INDEX idx_nameid ON Employee (Name, EmpID);
To create a unique index on EmpID column of a Employee table, the query is mentioned below:
CREATE UNIQUE INDEX idx_empid ON Employee (EmpID);
To create a unique index on multiple columns (Name and EmpID) of a table (Employee), the query is given below:
CREATE UNIQUE INDEX idx_empid ON Employee (Name, EmpID);
SQL Server DROP INDEX
The SQL Server DROP INDEX statement is used to delete a index on a table.
The syntax for using DROP INDEX statement in SQL Server (Transact-SQL) is given below:
DROP INDEX table_name.index_name;
SQL Server DROP INDEX Example
Consider a table called Employee which contains an index named idx_name. To drop this index following query can be used:
DROP INDEX Employee.idx_name;