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Introducing Keys As you can see in the previous example, we have purposely set up the three tables to link to one another. The EMPLOYEE table contains a column that has the department number that the employee belongs in. This department number also appears in the DEPARTMENT table, which describes each department in the company. The EMPLOYEE and CONFIDENTIAL tables are related, but we still need to add one corresponding entry (row) in one table for each entry in the other, the distinction coming from the employee s number. The link employee number and department number we have set up can be thought of as a key. A key is used to identify information within a table. Each individual employee or department should have a unique key to aid in various functions performed on the tables. In keeping with the relational model, the key is supposed to be unique within the table: No other entry in the table may have the same primary key. A single column is sometimes enough to uniquely identify a row, or entry. However, a combination of rows can be used to compose a primary key for example, we might want to just use the combination of the title and city location of a department to comprise the primary key. In SQL, columns defined as primary keys must be defined. They cannot be undefined (also known as NULL). Using Multiple Tables And Foreign Keys As we have shown, it s best to split data into tables so that the data contained within a table is logically associated. Oftentimes, the data will belong logically in more than one table, as is the case of the employee number in the EMPLOYEE and CONFIDENTIAL tables. We can further define that if a row in one table exists, a corresponding row must exist in another table; that is, we can say that if there is an entry in the EMPLOYEE table, there must be a corresponding entry in the CONFIDENTIAL table. We can solidify this association with the use of foreign keys, where a specific column in the dependent table matches a column in a parent table. In essence, we are linking a virtual column in one table to a real column in another table. In our example database, we link the CONFIDENTIAL table s employee number column to the employee number column in the EMPLOYEE table. We are also specifying that the employee number is a key in the CONFIDENTIAL table (hence the term foreign key). A composite primary key can contain a foreign key if necessary. We can create a logical structure to our data using the concept of a foreign key. However, in preparation, you ll have to put quite a bit of thought into creating your set of tables; an efficient and planned structure to the data by way of the tables and keys requires good knowledge of the data that is to be modeled. Unfortunately, a full discussion on the techniques of the subject is beyond the scope of this book. There are several different ways to efficiently model data; Figure 2.4 shows one visualization of the database we have created. The SQL queries we perform in the examples of this book are not very complex, so the information outlined in this section should suffice to convey a basic understanding of the example databases created throughout the following chapters.
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A word of caution: While the keywords are not case sensitive, the string values that are stored as data in a table do preserve case, as you would expect. Keep this in mind when doing string comparisons in queries. Putting It Into Perspective: Schema And Catalog Though you can stick all of your data into a single table, it doesn t make sense logically to do this all the time. For example, in our EMPLOYEE table shown previously, we could add information about company departments; however, the purpose of the EMPLOYEE table is to store data on the employees. The solution is for us to create another table, called DEPARTMENT, which will contain information about the specific departments in the company. To associate an employee with a department, we can simply add a column to the EMPLOYEE table that contains the department name or number. Now that we have employees and departments neatly contained, we can add another table, called PROJECT, to keep track of the projects each employee is involved in. Figure 2.2 shows our tables. Figure 2.2 The EMPLOYEE, DEPARTMENT, and PROJECT tables track employees by department and project. Now that you understand how to logically separate your data, it s time to take our model one step higher and introduce you to the schema/catalog relationship. The schema is a higher-level container that is defined as a collection of zero or more tables, where a table belongs to exactly one schema. In the same way, a catalog can contain zero or more schemas. This abstract is a necessary part of a robust relational database management system (RDBMS). The primary reason is access control: It facilitates who can read a table, who can change a table, and even who can create or destroy tables. Figure 2.3 demonstrates this point nicely. Here we have added another table, called CONFIDENTIAL. It contains the home address, home phone number, and salary of each employee. This information needs to belong in a separate schema so that anyone who is not in payroll cannot access the data, while allowing those in marketing to get the necessary data to do their job. Figure 2.3 The table, schema, and catalog relationship allows you to limit access to confidential information.
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A JDBC driver doesn t absolutely have to be SQL-92 compliant. The JDBC specification states the following: In order to pass JDBC compliance tests and to be called JDBC compliant, we require that a driver support at least ANSI SQL-92 Entry Level. This requirement is clearly not possible with drivers for legacy database management systems (DBMS). The driver in these cases will not implement all of the functions of a compliant driver. In Chapter 10, Writing JDBC Drivers, we develop the basics of a JDBC driver that implements only some of the features of SQL, but is a JDBC driver nonetheless. We ll start our exploration of SQL by discussing the relational model, the basis for SQL. Then we ll cover the essentials of building data tables using SQL. Finally, we ll go into the manipulation and extraction of the data from a datasource. The Relational Model And SQL Although SQL is based on the relational model, it is not a rigid implementation of it. In this section, we ll discuss the relational model as it pertains to SQL so we do not obfuscate our discussion of this standard, which is central to the JDBC specification. As part of its specification, the SQL-92 standard includes the definition of data types. We ll cover these data types, and how to map to Java, in Chapter 6, SQL Data Types in Java and the ORM. Understanding The Basics The basic units in SQL are tables, columns, and rows. So where does the relational model fit into the SQL units? Strictly speaking, in terms of the relation model, the relation is mapped in the table: It provides a way to relate the data contained within the table in a simple manner. A column represents a data element present in a table, while a row represents an instance of a record, or entry, in a table. Each row contains one specific value for each of the columns; a value can be blank or undefined and still be considered valid. The table can be visualized, you guessed it, as a matrix, with the columns being the vertical fields and the rows being the horizontal fields. Figure 2.1 shows an example table that can be used to store information about a company s employees. Figure 2.1 An SQL table. Before we push on, there are some syntax rules you need to be aware of: SQL is not whitespace sensitive. Carriage returns, tabs, and spaces don t have any special meaning when executing queries. Keywords and tokens are delimited by commas, when applicable, and parentheses are used for grouping. When performing multiple queries at one time, you must use semicolons to separate distinct queries. Queries are not case sensitive.
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Figure 1.2 ODBC in the JDBC model. ODBC drivers function in the same manner as true JDBC drivers; in fact, the JDBC- ODBC bridge is actually a sophisticated JDBC driver that does low-level translation to and from ODBC. When the JDBC driver for a certain database becomes available, you can easily switch from the ODBC driver to the new JDBC driver with few, if any, changes to the code of the Java program. Summary The JDBC is not only a specification for using data sources in Java applets and applications, but it also allows you to create and use low-level drivers to connect and talk with data sources. You have now explored the JDBC architecture and seen how the ODBC fits into the picture. The important concept to remember about the JDBC is that the modular design of the JDBC interface allows you to change between drivers hence databases without recoding your Java programs. In the next chapter, we ll take a step back to give you a quick primer on SQL, one of the pillars of the JDBC. If you are already familiar with SQL-92, feel free to skip the chapter. However, I think that you may find the chapter helpful in clarifying the SQL queries performed in the sample JDBC programs we develop in this book. Chapter 2 SQL 101 SQL the language of database. This chapter s primary purpose is to serve as a primer on this data sublanguage. Although it would be impossible for me to cover the intricacies of SQL in just one chapter, I do intend to give you a solid introduction that we ll build on in the remainder of this book. Because the JDBC requires that drivers support the ANSI SQL-92 standard to be JDBC compliant, I ll be basing this chapter on that standard. SQL-92, which I ll refer to as SQL, is based on the relational model of database management proposed in 1970 by Dr. E.F. Codd; over time, SQL evolved into the full- featured language it is today, and it continues to evolve with our ever-changing needs.
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ensures that the drivers will perform as expected. Essentially, the developer who has JDBC drivers for a certain database does not need to worry about changing the code for the Java program if a different type of database is used (assuming that the JDBC driver for the other database is available). This is especially useful in the scenario of distributed databases. Figure 1.1 The architecture of the JDBC. The JDBC uses a URL syntax for specifying a database. For example, a connection to a mSQL database, which was used to develop some of the Java applets in this book, is: jdbc:msql://mydatabase.server.com:1112/testdb This statement specifies the transport to use (jdbc), the database type (msql), the server name, the port (1112), and the database to connect to (testdb). We ll discuss specifying a database more thoroughly in Chapter 3. The data types in SQL are mapped into native Java types whenever possible. When a native type is not present in Java, a class is available for retrieving data of that type. Consider, for example, the Date type in the JDBC. A developer can assign a date field in a database to a JDBC Date class, after which the developer can use the methods in the Date class to display or perform operations. The JDBC also includes support for binary large objects, or BLOB data types; you can retreive and store images, sound, documents, and other binary data in a database with the JDBC. In Chapter 6, we ll cover the SQL data types and their mapping into Java/JDBC, as well object-relational mapping. ODBC s Part In The JDBC The JDBC and ODBC share a common parent: Both are based on the same X/OPEN call level interface for SQL. Though there are JDBC drivers emerging for many databases, you can write database-aware Java programs using existing ODBC drivers. In fact, Javasoft and Intersolv have written a JDBC driver the JDBC-ODBC Bridge that allows developers to use exisiting ODBC drivers in Java programs. Figure 1.2 shows the place of the JDBC-ODBC Bridge in the overall architecture of the JDBC. However, the JDBC-ODBC Bridge requires pre-installation on the client, or wherever the Java program is actually running, because the Bridge must make native method calls to do the translation from ODBC to JDBC. This pre-installation issue is also true for JDBC drivers that use native methods. Only 100 percent Java JDBC drivers can be downloaded across a network with a Java applet, thus requiring no pre-installation of the driver.
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RogueWave Software, Inc. SAS Institute, Inc. SCO Sybase, Inc. Symantec Thunderstone Visigenic Software, Inc. WebLogic, Inc. XDB Systems, Inc. The JDBC is heavily based on the ANSI SQL-92 standard, which specifies that a JDBC driver should be SQL-92 entry-level compliant to be considered a 100 percent JDBC- compliant driver. This is not to say that a JDBC driver has to be written for an SQL-92 database; a JDBC driver can be written for a legacy database system and still function perfectly. As a matter of fact, the simple JDBC driver included with this book uses delimited text files to store table data. Even though the driver does not implement every single SQL-92 function, it is still a JDBC driver. This flexibility will be a major selling point for developers who are bound to legacy database systems but who still want to extend their client applications. The JDBC Structure As I mentioned at the beginning of this chapter, the JDBC is two-dimensional. The reasoning for the split is to separate the low-level programming from the high-level application interface. The low-level programming is the JDBC driver. The idea is that database vendors and third-party software vendors will supply pre-built drivers for connecting to different databases. JDBC drivers are quite flexible: They can be local data sources or remote database servers. The implementation of the actual connection to the data source/database is left entirely to the JDBC driver. The structure of the JDBC includes these key concepts: The goal of the JDBC is a DBMS independent interface, a generic SQL database access framework, and a uniform interface to different data sources. The programmer writes only one database interface; using JDBC, the program can access any data source without recoding. Figure 1.1 shows the architecture of the JDBC. The DriverManager class is used to open a connection to a database via a JDBC driver, which must register with the DriverManager before the connection can be formed. When a connection is attempted, the DriverManager chooses from a given list of available drivers to suit the explict type of database connection. After a connection is formed, the calls to query and fetch results are made directly with the JDBC driver. The JDBC driver must implement the classes to process these functions for the specific database, but the rigid specification of the JDBC
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It s an API for using low-level JDBC drivers. It s an API for creating the low-level JDBC drivers, which do the actual connecting/transacting with data sources. It s based on the X/Open SQL Call Level Interface (CLI) that defines how client/server interactions are implemented for database systems. Confused yet? It s really quite simple: The JDBC defines every aspect of making data- aware Java applications and applets. The low-level JDBC drivers perform the database- specific translation to the high-level JDBC interface. This interface is used by the developer so he doesn t need to worry about the database-specific syntax when connecting to and querying different databases. The JDBC is a package, much like other Java packages such as java.awt. It s not currently a part of the standard Java Developer s Kit (JDK) distribution, but it is slated to be included as a standard part of the general Java API as the java.sql package. Soon after its official incorporation into the JDK and Java API, it will also become a standard package in Java-enabled Web browsers, though there is no definite timeframe for this inclusion. The exciting aspect of the JDBC is that the drivers necessary for connection to their respective databases do not require any pre- installation on the clients: A JDBC driver can be downloaded along with an applet! The JDBC project was started in January of 1996, and the specification was frozen in June of 1996. Javasoft sought the input of industry database vendors so that the JDBC would be as widely accepted as possible when it was ready for release. And, as you can see from this list of vendors who have already endorsed the JDBC, it s sure to be widely accepted by the software industry: Borland International, Inc. Bulletproof Cyber SQL Corporation DataRamp Dharma Systems, Inc. Gupta Corporation IBM s Database 2 (DB2) Imaginary (mSQL) Informix Software, Inc. Intersoft Intersolv Object Design, Inc. Open Horizon OpenLink Software Oracle Corporation Persistence Software Presence Information Design PRO-C, Inc. Recital Corporation
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public interface PreparedStatement public interface ResultSet public interface ResultSetMetaData public interface Statement Exceptions public class DataTruncation public class SQLException public class SQLWarning Appendix A Appendix B Appendix C Appendix D Index Chapter 1 JDBC: Databases The Java Way! The Internet has spurred the invention of several new technologies in client/server computing the most recent of which is Java. Java is two-dimensional: It s a programming language and also a client/server system in which programs are automatically downloaded and run on the local machine (instead of the server machine). The wide embrace of Java has prompted its quick development. Java includes Java compilers, interpreters, tools, libraries, and integrated development environments (IDEs). Javasoft is leading the way in the development of libraries to extend the functionality and usability of Java as a serious platform for creating applications. One of these libraries, called Application Programming Interfaces (APIs), is the Java Database Connectivity API, or JDBC. Its primary purpose is to intimately tie connectivity to databases with the Java language. We ll discuss the reasoning behind the JDBC in this chapter, as well as the design of the JDBC and its associated API. The Internet, or better yet, the technologies used in the operation of the Internet, are tied into the design of the JDBC. The other dominant design basis for the JDBC is the database standard known as SQL. Hence, the JDBC is a fusion of three discrete computer areas: Java, Internet technology, and SQL. With the growing implementation of these Internet technologies in closed networks, called intranets, the time was right for the development of Java-based enterprise APIs. In this book, intranet and Internet are both used to describe the software technology behind the network, such as the World Wide Web. What Is The JDBC? As I mentioned a moment ago, JDBC stands for Java Database Connectivity. What is this JDBC besides a nifty acronym? It refers to several things, depending on context: It s a specification for using data sources in Java applets and applications.
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Writing Tracing Information Checking For Tracing Data Coercion Escape Clauses Date, Time, And Timestamp Scalar Functions LIKE Predicate Escape Characters Outer Joins Procedures The JDBC Interfaces Driver Connection DatabaseMetaData Statement PreparedStatement ResultSet ResultSetMetaData Summary Chapter 11 Internet Database Issues: Middleware Connectivity Issues Involved With Database Access Advantages Of Middleware Disadvantages Of Middleware The Application Server: A Complete Example With Code The Client: A Complete Example With Code Summary Chapter 12 The JDBC API Classes public class Date public class DriverManager public class DriverPropertyInfo public final class Numeric public class Time public class TimeStamp public class Types Interfaces public interface CallableStatement public interface Connection public interface DatabaseMetaData public interface Driver
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Establishing The Database Connection Creating The Menu Creating The Lists Handling Events Saving The Image Summary Chapter 9 Java And Database Security Database Server Security Rooting Out The Packet Sniffers Web Server CGI Holes Finding A Solution Applet Security: Can I Trust You? The Applet Security Manager I m A Certified Applet Summary Chapter 10 Writing Database Drivers The JDBC Driver Project: SimpleText SimpleText SQL Grammar SimpleText File Format The DriverManager JDBC Exception Types JDBC Data Types Character Data: CHAR, VARCHAR, And LONGVARCHAR Exact Numeric Data: NUMERIC And DECIMAL Binary Data: BINARY, VARBINARY, And LONGVARBINARY Boolean Data: BIT Integer Data: TINYINT, SMALLINT, INTEGER, And BIGINT Floating-Point Data: REAL, FLOAT, And DOUBLE Time Data: DATE, TIME, And TIMESTAMP New Data Classes Numeric Date Time Timestamp Native Drivers: You re Not From Around Here, Are Ya? Implementing Interfaces Tracing Turning On Tracing
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