An index in a table is nothing but a pointer to its data. These are used to speed up the data retrieval from a table.

If we use indexes, the INSERT and UPDATE statements get executed in a slower phase. Whereas SELECT and WHERE get executed with in lesser time.

Creating an Index

The CREATE INDEX statement is used for creating a new Index in a table in Derby database.

Syntax

Following is the syntax of the CREATE INDEX statement −

CTREATE INDEX index_name on table_name (column_name);

Example

Suppose we have created a table named Employees in Apache Derby as shown below.

CREATE TABLE Emp ( Id INT NOT NULL GENERATED ALWAYS AS IDENTITY,
   Name VARCHAR(255),
   Salary INT NOT NULL,
   Location VARCHAR(255),
   Phone_Number BIGINT
);

The following SQL statement creates an index on the column named Salary in the table Employees.

ij> CREATE INDEX example_index on Emp (Salary);
0 rows inserted/updated/deleted

Creating a UNIQUE index

In Apache Derby, UNIQUE indexes are used for data integration. Once you create a UNIQUE index on a column in a table, it does not allow duplicate values.

Syntax

Following is the syntax of creating a unique index.

CREATE UNIQUE INDEX index_name on table_name (column_name);

Example

Following example creates a UNIQUE index on the column Id of the table Employee.

ij> CREATE UNIQUE INDEX unique_index on Emp (Phone_Number);
0 rows inserted/updated/deleted

Once you have created a unique index on a column, you cannot enter same values for that column in another row. In short, a column which is has a UNIQE index will not allow duplicate values.

Insert a row in the Emp table as shown below

ij> INSERT INTO Emp(Name, Salary, Location, Phone_Number) VALUES 
   (''Amit'', 45000, ''Hyderabad'', 9848022338);
1 row inserted/updated/deleted

Since we have created a unique index on the column Phone_No, if you ty to enter the same value as in the previous record, it shows an error.

ij> INSERT INTO Emp(Name, Salary, Location, Phone_Number) VALUES 
   (''Sumit'', 35000, ''Chennai'', 9848022338);
ERROR 23505: The statement was aborted because it would have caused a duplicate
key value in a unique or primary key constraint or unique index identified by
''UNIQUE_INDEX'' defined on ''EMP''.

Creating a COMPOSITE index

You can create a single index on two rows and it is called Composite index.

Syntax

Following is the syntax of the composite index.

CREATE INDEX index_name on table_name (column_name1, column_name2);

Example

Following index creates a composite index on the columns Name and Location.

ij> CREATE INDEX composite_index on Emp (Name, Location);
0 rows inserted/updated/deleted

Displaying the Indexes

The SHOW INDEXES query displays the list of indexes on a table.

Syntax

Following is the syntax of the SHOW INDEXES statement −

SHOW INDEXES FROM table_name;

Example

Following example, i displays the indexes on the table Employees.

ij> SHOW INDEXES FROM Emp;

This produces the following result.

ij> SHOW INDEXES FROM Emp;
TABLE_NAME |COLUMN_NAME |NON_U&|TYPE|ASC&|CARDINA&|PAGES
----------------------------------------------------------------------------
EMP        |PHONE_NUMBER|false |3   |A   |NULL    |NULL
EMP        |NAME        |true  |3   |A   |NULL    |NULL
EMP        |LOCATION    |true  |3   |A   |NULL    |NULL
EMP        |SALARY      |true  |3   |A   |NULL    |NULL
4 rows selected

Dropping Indexes

The Drop Index statement deletes/drops the given index on a column.

Syntax

Following is the syntax of the DROP INDEX statement.

DROP INDEX index_name;

Example

Following example drops an indexes named composite_index and unique_index created above.

ij> DROP INDEX composite_index;
0 rows inserted/updated/deleted
ij>Drop INDEX unique_index;
0 rows inserted/updated/deleted

Now, if you verify the list of indexes you can see index on one column since we have deleted the remaining.

ij> SHOW INDEXES FROM Emp;
TABLE_NAME |COLUMN_NAME |NON_U&|TYPE|ASC&|CARDINA&|PAGES
----------------------------------------------------------------------------
EMP        |SALARY      |true  |3   |A   |NULL    |NULL
1 row selected

Handling Indexes using JDBC program

Following JDBC program demonstrates how to create drop indexes on a column in a table.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
public class IndexesExample {
   public static void main(String args[]) throws Exception {
      //Registering the driver
      Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
      //Getting the Connection object
      String URL = "jdbc:derby:MYDATABASE;create=true";
      Connection conn = DriverManager.getConnection(URL);

      //Creating the Statement object
      Statement stmt = conn.createStatement();

      //Creating the Emp table
      String createQuery = "CREATE TABLE Emp( "
         + "Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, "
         + "Name VARCHAR(255), "
         + "Salary INT NOT NULL, "
         + "Location VARCHAR(255), "
         + "Phone_Number BIGINT )";
      stmt.execute(createQuery);
      System.out.println("Table created");
      System.out.println(" ");

      //Creating an Index on the column Salary
      stmt.execute("CREATE INDEX example_index on Emp (Salary)");
      System.out.println("Index example_index inserted");
      System.out.println(" ");

      //Creating an Unique index on the column Phone_Number
      stmt.execute("CREATE UNIQUE INDEX unique_index on Emp (Phone_Number)");
      System.out.println("Index unique_index inserted");
      System.out.println(" ");

      //Creating a Composite Index on the columns Name and Location
      stmt.execute("CREATE INDEX composite_index on Emp (Name, Location)");
      System.out.println("Index composite_index inserted");
      System.out.println(" ");

      //listing all the indexes
      System.out.println("Listing all the columns with indexes");

      //Dropping indexes
      System.out.println("Dropping indexes unique_index and, composite_index ");
      stmt.execute("Drop INDEX unique_index");
      stmt.execute("DROP INDEX composite_index");
   }
}

Output

On executing, this generates the following result

Table created
Index example_index inserted
Index unique_index inserted
Index composite_index inserted

Listing all the columns with indexes
Dropping indexes unique_index and, composite_index