JAVA DATABASE CONNECTIVITY (JDBC):

from http://java.sun.com/products/jdk/1.1/docs/guide/jdbc/getstart/intro.doc.html#1005262

Simply put, JDBC makes it possible to do three things:

establish a connection with a database

send SQL statements

process the results.

The following code fragment gives a basic example of these three steps:

Connection con = DriverManager.getConnection (

"jdbc:odbc:wombat", "login", "password");

Statement stmt = con.createStatement();

ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM Table1");

while (rs.next()) {

int x = rs.getInt("a");

String s = rs.getString("b");

float f = rs.getFloat("c");

}

JDBC is a "low-level" interface, which means that it is used to invoke (or "call") SQL commands directly. It works very well in this capacity and is easier to use than other database connectivity APIs, but it was designed also to be a base upon which to build higher-level interfaces and tools. A higher-level interface is "user-friendly," using a more understandable or more convenient API that is translated behind the scenes into a low-level interface such as JDBC.

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At this point, Microsoft's ODBC (Open DataBase Connectivity) API is probably the most widely used programming interface for accessing relational databases. It offers the ability to connect to almost all databases on almost all platforms. So why not just use ODBC from Java?

The answer is that you can use ODBC from Java, but this is best done with the help of JDBC in the form of the JDBC-ODBC Bridge, which we will cover shortly.

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The JDBC API supports both two-tier and three-tier models for database access.

In the two-tier model, a Java applet or application talks directly to the database. This requires a JDBC driver that can communicate with the particular database management system being accessed. A user's SQL statements are delivered to the database, and the results of those statements are sent back to the user. The database may be located on another machine to which the user is connected via a network. This is referred to as a client/server configuration, with the user's machine as the client, and the machine housing the database as the server.

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In the three-tier model, commands are sent to a "middle tier" of services, which then send SQL statements to the database. The database processes the SQL statements and sends the results back to the middle tier, which then sends them to the user. MIS directors find the three-tier model very attractive because the middle tier makes it possible to maintain control over access and the kinds of updates that can be made to corporate data. Another advantage is that when there is a middle tier, the user can employ an easy-to-use higher-level API [ with its GUIs on the client machine ] which is translated by the middle tier into the appropriate low-level calls. Finally, in many cases the three-tier architecture can provide performance advantages.