The GROUP BY clause is used with SELECT statements. It is used to form subsets in case of identical data. Usually, this clause is followed by ORDER BY clause and placed after the WHERE clause.
Syntax
Following is the syntax of GROUP BY clause −
ij>SELECT column1, column2, . . . table_name GROUP BY column1, column2, . . .;
Example
Suppose we have a table named Employees in the database with the following records −
ID |NAME |SALARY |LOCATION ------------------------------------------------------------------ 1 |Amit |30000 |Hyderabad 2 |Rahul |39000 |Lucknow 3 |Renuka |50000 |Hyderabad 4 |Archana |15000 |Vishakhapatnam 5 |Kalyan |40000 |Hyderabad 6 |Trupthi |45000 |Vishakhapatnam 7 |Raghav |12000 |Lucknow 8 |Suchatra|33000 |Vishakhapatnam 9 |Rizwan |20000 |Lucknow
The following SELECT statement with GROUP BY clause groups the table based on location. It displays the total amount of salary given to employees at a location.
ij> SELECT Location, SUM(Salary) from Employees GROUP BY Location;
This will generate the following output −
LOCATION |2 ------------------------------------------------------- Hyderabad |120000 Lucknow |71000 Vishakhapatnam |93000 3 rows selected
In the same way, following query finds the average amount spent on the employees as salary in a location.
ij> SELECT Location, AVG(Salary) from Employees GROUP BY Location;
This will generate the following output −
LOCATION |2 ----------------------------------------------------- Hyderabad |40000 Lucknow |23666 Vishakhapatnam |31000 3 rows selected
Group By clause JDBC example
This section teaches you how to use Group By clause and perform CURD operations on a table in Apache Derby database using JDBC application.
If you want to request the Derby network server using network client, make sure that the server is up and running. The class name for the Network client driver is org.apache.derby.jdbc.ClientDriver and the URL is jdbc:derby://localhost:1527/DATABASE_NAME;create=true;user=USER_NAME;password=PASSWORD“
Follow the steps given below to use Group By clause and perform CURD operations on a table in Apache Derby
Step 1: Register the driver
To communicate with the database, first of all, you need to register the driver. The forName() method of the class Class accepts a String value representing a class name loads it in to the memory, which automatically registers it. Register the driver using this method.
Step 2: Get the connection
In general, the first step we do to communicate to the database is to connect with it. The Connection class represents the physical connection with a database server. You can create a connection object by invoking the getConnection() method of the DriverManager class. Create a connection using this method.
Step 3: Create a statement object
You need to create a Statement or PreparedStatement or, CallableStatement objects to send SQL statements to the database. You can create these using the methods createStatement(), prepareStatement() and, prepareCall() respectively. Create either of these objects using the appropriate method.
Step 4: Execute the query
After creating a statement, you need to execute it. The Statement class provides various methods to execute a query like the execute() method to execute a statement that returns more than one result set. The executeUpdate() method is used to execute queries like INSERT, UPDATE, DELETE. The executeQuery() method returns data. Use either of these methods and execute the statement created previously.
Example
Following JDBC example demonstrates how to use Group By clause and perform CURD operations on a table in Apache Derby using JDBC program. Here, we are connecting to a database named sampleDB (will create if it does not exist) using the embedded driver.
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
public class GroupByClauseExample {
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:sampleDB;create=true";
Connection conn = DriverManager.getConnection(URL);
//Creating the Statement object
Statement stmt = conn.createStatement();
//Creating a table and populating it
stmt.execute("CREATE TABLE EmployeesData( "
+ "Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, "
+ "Name VARCHAR(255), "
+ "Salary INT NOT NULL, "
+ "Location VARCHAR(255), "
+ "PRIMARY KEY (Id))");
stmt.execute("INSERT INTO EmployeesData(Name, Salary, Location) "
+ "VALUES ('Amit', 30000, 'Hyderabad'), "
+ "('Rahul', 39000, 'Lucknow'), "
+ "('Renuka', 50000, 'Hyderabad'), "
+ "('Archana', 15000, 'Vishakhapatnam'), "
+ "('Kalyan', 40000, 'Hyderabad'), "
+ "('Trupthi', 45000, 'Vishakhapatnam'), "
+ "('Raghav', 12000, 'Lucknow'), "
+ "('Suchatra', 33000, 'Vishakhapatnam'), "
+ "('Rizwan', 20000, 'Lucknow')");
//Executing the query
String query = "SELECT Location, SUM(Salary) from EmployeesData GROUP BY Location";
ResultSet rs = stmt.executeQuery(query);
while(rs.next()) {
System.out.println("Location: "+rs.getString(1));
System.out.println("Sum of salary: "+rs.getString(2));
System.out.println(" ");
}
}
}
Output
On executing the above program, you will get the following output −
Location: Hyderabad Sum of salary: 120000 Location: Lucknow Sum of salary: 71000 Location: Vishakhapatnam Sum of salary: 93000