1. We imagine in the above that the client is a web browser, or is a mobile app making a request to a web server for a HTML, XML, or JSON object.
2. The web server are processes that connect clients to the database system usually over the Internet, but possibly over a local connect. These serve web pages (HTML, XML, JSON) (the View in MVC) based on the particular request of the user.
3. The Application Server are processes that perform the "business logic" for the application program. For example, it might handle the logic of who has bought what. In MVC, this logic might correspond to controller classes. Applications servers might be implemented using servlet technology, fast CGI (common gateway interface), etc.
4. The Database Server run the processes of the DBMS and can perform queries and modification to the database at the request of the application servers. They are used to handle model requests in the MVC pattern.

1. Physical Layer. This layer uses electrons/photons to transmit bits down a communication line.
2. Link/Host-to-Network Layer. This handles the error correction, etc needed to provide using the physical communication of frames (bounded length sequences of bits) between two machines or between a machine and a switch/router all on the same line. If more than one machine is on the same line a protocol such as Ethernet might might be used to say which machine is what.
3. Internet Layer. This handles communicating packets between two different machines over the internet. These machines are not assumed to be on the same line or local area network. We imagine the internet is being built out of machines connected to routers and routers connected to other routers and further machines. Machines in this network are identified by IP (Internet Protocol) addresses. For example, 173.13.143.73. Two common standard for these addresses are IPv4 and IPv6.
4. Transport Layer. This layers uses packets to fake having a direct connection (using TCP (Transfer Control Protocol)) between two processes on different machines over which they can communicate sequences of bytes. To say which process we are communicating between we use a two byte port number. For example, we might say we want to communicate with 80 on a machine to access its web server or port 3306 to access its Mysql server.
5. Application Layer. These the protocols that web servers, database servers, etc. use to communicate via connections from the transport layer.

CREATE DATABASE test;

CREATE TABLE my_table (
   a INT,
   b VARCHAR(20)
);
INSERT INTO my_table VALUES
(1, "hi"), (2, "bye"), (3, "yo"); 

import java.sql.*;

class JdbcTest
{
    public static void main(String[] args)
    {
        Connection conn = null;
        ResultSet rs = null;
        Statement stmt = null;
        try {
            conn = DriverManager.getConnection(
                "jdbc:mysql://localhost/test?" +
                "user=root&password=root");
            stmt = conn.createStatement();
            rs = stmt.executeQuery("SELECT * FROM my_table");
            while (rs.next()) {
                int a = rs.getInt("a");
                String b = rs.getString("b");
                System.out.println("a:" + a + "\tb:" + b);
            }
        } catch (SQLException ex) {
            // handle any errors
            System.out.println("SQLException: " + ex.getMessage());
            System.out.println("SQLState: " + ex.getSQLState());
            System.out.println("VendorError: " + ex.getErrorCode());
        } finally {
            if (rs != null) {
                try {
                    rs.close();
                } catch (SQLException sqlEx) { } // ignore
                rs = null;
            }
            if (stmt != null) {
                try {
                    stmt.close();
                } catch (SQLException sqlEx) { } // ignore
                stmt = null;
            }
        }
    }
}

conn = DriverManager.getConnection(
   jdbc:mysql://localhost/test?" +
   "user=root&password=root");

javac JdbcTest.java

java -cp .:/Users/cpollett/Desktop/mysql-connector-j-8.2.0.jar JdbcTest

a:1	b:hi
a:2	b:bye
a:3	b:yo

int a=5;
rs = stmt.executeQuery("SELECT * FROM my_table WHERE a=" + a);

PreparedStatement pstmt = conn.prepareStatement
    ("select * from EMPLOYEE where fname=? and salary=?");
pstmt.setString(1,"bob");
pstmt.setInt(2, 123);
ResultSet rs = pstmt.executeQuery();

String ssn  = "123456789";
stmt.executeUpdate("DELETE FROM EMPLOYEE WHERE SSN="+ssn);