Retrieve data example

This is an example of how to retrieve data from a database. Retrieving data from a database implies that you should:

• Load the JDBC driver, using the forName(String className) API method of the Class. In this example we use the MySQL JDBC driver.
• Create a Connection to the database. Invoke the getConnection(String url, String user, String password) API method of the DriverManager to create the connection.
• Create a Statement, using the createStatement() API method of the Connection.
• Execute the query to the database, using the executeQuery(String sql) API method. The data produced by the given query is a ResultSet.
• For each row of the result set, get the data of a column, using the next() and the getString(String columnLabel) API methods of the ResultSet. Note that ResultSet API provides appropriate methods for retrieving data, according to the datatype, such as getBoolean(String columnLabel), getByte(String columnLabel), getShort(String columnLabel), getDouble(String columnLabel), getDate(String columnLabel). We can also get the data from the current row using the column index with the getString(int columnIndex) API method.

Let’s take a look at the code snippet that follows:

package com.javacodegeeks.snippets.core;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class SelectRowsExample {
 
  public static void main(String[] args) {
    Connection connection = null;
    try {
  // Load the MySQL JDBC driver
  String driverName = "com.mysql.jdbc.Driver";
  Class.forName(driverName);
  // Create a connection to the database
  String serverName = "localhost";
  String schema = "test";
  String url = "jdbc:mysql://" + serverName +  "/" + schema;
  String username = "username";
  String password = "password";
  connection = DriverManager.getConnection(url, username, password);
  
  System.out.println("Successfully Connected to the database!");
  
    } catch (ClassNotFoundException e) {
  System.out.println("Could not find the database driver " + e.getMessage());
    } catch (SQLException e) {
  System.out.println("Could not connect to the database " + e.getMessage());
    }
    try {
// Get a result set containing all data from test_table
Statement statement = connection.createStatement();
ResultSet results = statement.executeQuery("SELECT * FROM test_table");
// For each row of the result set ...
while (results.next()) {
  // Get the data from the current row using the column index - column data are in the VARCHAR format
  String data = results.getString(1);
  System.out.println("Fetching data by column index for row " + results.getRow() + " : " + data);
  // Get the data from the current row using the column name - column data are in the VARCHAR format
  data = results.getString("test_col");
  System.out.println("Fetching data by column name for row " + results.getRow() + " : " + data);
}
/*
  * Please note :
  * ResultSet API provides appropriate methods for retrieving data 
  * based on each column data type e.g.
  * 
  * boolean bool = rs.getBoolean("test_col");
  * byte b = rs.getByte("test_col");
  * short s = rs.getShort("test_col");
  * int i = rs.getInt("test_col");
  * long l = rs.getLong("test_col");
  * float f = rs.getFloat("test_col");
  * double d = rs.getDouble("test_col");
  * BigDecimal bd = rs.getBigDecimal("test_col");
  * String str = rs.getString("test_col");
  * Date date = rs.getDate("test_col");
  * Time t = rs.getTime("test_col");
  * Timestamp ts = rs.getTimestamp("test_col");
  * InputStream ais = rs.getAsciiStream("test_col");
  * InputStream bis = rs.getBinaryStream("test_col");
  * Blob blob = rs.getBlob("test_col");
  */
    } catch (SQLException e) {
  System.out.println("Could not retrieve data from the database " + e.getMessage());
    }
  }
}

Example Output:

Successfully Connected to the database!
Fetching data by column index for row 1 : new_test_value
Fetching data by column name for row 1 : new_test_value
Fetching data by column index for row 2 : new_test_value_0
Fetching data by column name for row 2 : new_test_value_0
Fetching data by column index for row 3 : new_test_value_1
Fetching data by column name for row 3 : new_test_value_1
Fetching data by column index for row 4 : new_test_value_2
Fetching data by column name for row 4 : new_test_value_2
Fetching data by column index for row 5 : new_test_value_3
Fetching data by column name for row 5 : new_test_value_3
Fetching data by column index for row 6 : new_test_value_4
Fetching data by column name for row 6 : new_test_value_4
Fetching data by column index for row 7 : new_test_value_5
Fetching data by column name for row 7 : new_test_value_5
Fetching data by column index for row 8 : new_test_value_6
Fetching data by column name for row 8 : new_test_value_6
Fetching data by column index for row 9 : new_test_value_7
Fetching data by column name for row 9 : new_test_value_7
Fetching data by column index for row 10 : new_test_value_8
Fetching data by column name for row 10 : new_test_value_8
Fetching data by column index for row 11 : new_test_value_9
Fetching data by column name for row 11 : new_test_value_9

 
This was an example of how to retrieve data from a database in Java.