SQL Create Index

SQL CREATE INDEX Statement

The CREATE INDEX statement is used to create indexes on tables in databases, to speed up data retrieval.

The users cannot see the indexes, they are just used to speed up searches/queries.

Note: Updating tables with indexes are more time-consuming than tables without indexes (because the indexes must also be updated). So, only create indexes on columns that are frequently searched against.

Types of Indexes: Non-unique and Unique

There are two types of indexes:

• CREATE INDEX - Creates a non-unique index (duplicate values are allowed)
• CREATE UNIQUE INDEX - Creates a unique index (duplicate values are not allowed)

CREATE INDEX Syntax

CREATE INDEX index_name

ON table_name (column1, column2, ...);

CREATE UNIQUE INDEX Syntax

CREATE UNIQUE INDEX index_name

ON table_name (column1, column2, ...);

Note: The syntax for creating indexes varies among different databases. Check the syntax for creating indexes in your database!

CREATE INDEX Example

The following SQL creates a non-unique index named "idx_lastname" on the "LastName" column in the "Persons" table:

CREATE INDEX idx_lastname

ON Persons (LastName);

If you want to create an index on a combination of columns, you can list the column names within the parentheses, separated by commas:

CREATE INDEX idx_lname_fname

ON Persons (LastName, FirstName);

DROP INDEX Statement

The DROP INDEX statement is used to remove an index.

SQL Server:

DROP INDEX table_name.index_name;

MySQL:

ALTER TABLE table_name
DROP INDEX index_name;

MS Access:

DROP INDEX index_name ON table_name;

DB2/Oracle:

DROP INDEX index_name;