Tài liệu Java Database Programming Bible- P5 docx

Chapter 6:Inserting, Updating, and Deleting Data -199- Using INSERT SELECT The INSERT statement illustrated in the example of Listing 6-1 is primarily intended for inserting records into a table one at a time. For applications such as storing information from membership applications or entering employee records, this is the perfect solution. However, there are times when you want to copy subsets of data from one table to another. On these occasions, doing the transfer one record at a time introduces a lot of overhead because each record has to be individually retrieved from one table and inserted into another other. SQL allows you to handle these situations by combining the INSERT command with a SELECT command, which queries the database for the desired records. The advantage of this approach is that the whole process is carried out within the RDBMS, avoiding the overhead of retrieving records and reinserting them externally. The SELECT statement The SELECT statement is used to query the database for specific rows. This is the basic form of the SELECT statement: SELECT Field1, Field2, FROM TableName [ WHERE ]; In place of a comma-delimited list of field names, you can supply the asterisk wildcard character, *, to request all fields: SELECT * FROM TableName; Cross-Reference The SELECT statement is discussed in detail in Chapter 7. An example of a situation where you might use INSERT SELECT is the creation of a table containing only the first and last names from the Contact_Info Table. As illustrated in Chapter 5, the SQL command to create the table is as follows: CREATE TABLE Names (First_Name VARCHAR(20), Last_Name LName VARCHAR(30)); To insert the corresponding data from the original Contact_Info Table, use a SQL INSERT SELECT command to select the desired fields from the Contact_Info Table, and insert them into the new Names Table. Here's an example: INSERT INTO Names SELECT First_Name, Last_Name FROM Contact_Info; Essentially, this command tells the database management system to perform these two separate operations internally: TEAMFLY Team-Fly ® Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -200- A SELECT (to query the Contact_Info Table for the FName and LName fields from all records) An INSERT (to input the resulting record set into the new Names Table By performing these operations within the RDBMS, the use of the INSERT SELECT command eliminates the overhead of retrieving the records and reinserting them. The WHERE clause The optional WHERE clause allows you to make conditional queries; for example, you can get all records where the last name is "Corleone" and insert them into the Names Table with this statement: INSERT INTO Names SELECT First_Name, Last_Name FROM Contact_Info WHERE Last_Name = 'Corleone'; Using INSERT SELECT with JDBC As with any other SQL command, it is easy to use INSERT SELECT with JDBC. If you substitute the code snippet of Listing 6-2 for the main() of Listing 6-1 and run the example again, you will create a Name Table populated with the first and last names. Listing 6-2: Using INSERT SELECT with JDBC public static void main(String args[]){ DataInserter inserter = new DataInserter(); String SQLCommand = "INSERT INTO NAMES "+ "SELECT First_Name,Last_Name FROM CONTACT_INFO "+ "WHERE Last_Name = 'Corleone'; "; inserter.execute(SQLCommand); } } Once you have data in a table, you are likely to have to update it to reflect changes in data fields like addresses or inventory item count. The next section shows how to use the SQL UPDATE command to modify data in a table. The UPDATE Statement A frequent requirement in database applications is updating records. For example, when a contact moves, you need to change his or her address. Do this with the SQL UPDATE statement, using a WHERE clause to identify the record you want to change. Here's an example: Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -201- UPDATE Contact_Info SET Street = '55 Broadway', ZIP = '10006' WHERE First_Name = 'Michael' AND Last_Name = 'Corleone'; This statement first evaluates the WHERE clause to find all records with matching First_Name and Last_Name. It then makes the address change to all of those records. Caution If you omit the WHERE clause from the UPDATE statement, all records in the given table are updated. Using Calculated Values with UPDATE You can also use the UPDATE statement to update columns with calculated values. For example, if you add stock to your inventory, instead of setting the Qty column to an absolute value, you can simply add the appropriate number of units with a calculated UPDATE statement like this: UPDATE Inventory SET Qty = QTY + 24 WHERE Name = 'Corn Flakes'; When you use a calculated UPDATE statement like this, you need to make sure that you observe the rules for INSERTS and UPDATES mentioned earlier. In particular, ensure that the data type of the calculated value is the same as the data type of the field you are modifying, as well as short enough to fit in the field. Common Problems with UPDATE Two common problems can result from the use of calculated values: § Truncation can result from number conversions, such as conversion from a real number to an integer. § Overflow occurs when the resulting value is larger than the capacity of the column. This causes the database system to return an error. Problems of this type can be avoided if you observe the rules for INSERTS and UPDATES mentioned earlier. Listing 6-3: Using UPDATE with JDBC package jdbc_bible.part2; import java.awt.event.*; import java.sql.*; import sun.jdbc.odbc.JdbcOdbcDriver; Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -202 - public class DataUpdater{ static String jdbcDriver = "sun.jdbc.odbc.JdbcOdbcDriver"; static String dbName = "Contacts"; static String urlRoot = "jdbc:odbc:"; private ActionListener exceptionListener = null; public DataUpdater(){ registerDriver(); } public void setDatabaseName(String dbName){ this.dbName=dbName; } public void registerDriver(){ try { Class.forName(jdbcDriver); DriverManager.registerDriver(new JdbcOdbcDriver()); } catch(ClassNotFoundException e){ System.err.println(e.getMessage()); } catch(SQLException e){ System.err.println(e.getMessage()); } } public void execute(String SQLCommand){ String url = urlRoot+dbName; try { Connection con = DriverManager.getConnection(url); Statement stmt = con.createStatement(); stmt.execute(SQLCommand); con.close(); } catch(SQLException e){ System.err.println(e.getMessage()); Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -203 - } } public static void main(String args[]){ DataUpdater inserter = new DataUpdater(); String SQLCommand = "UPDATE CONTACT_INFO "+ "SET STREET = '58 Broadway', ZIP = '10008' "+ "WHERE First_Name = 'Michael' AND "+ "Last_Name ='Corleone';"; inserter.execute(SQLCommand); } } Once again, the basic Java code used to issue the SQL command remains unchanged. To try it out, compile and execute the example; you should be able to see the modified record in your Contact_Info Table. Transaction Management with COMMIT and ROLLBACK Transaction management refers to the capability of a relational database management system to execute database commands in groups, known as transactions. A transaction is a group or sequence of commands, all of which must be executed in order and must complete successfully. If anything goes wrong during the transaction, the database management system will allow the entire transaction to be cancelled or "rolled back." If, on the other hand, it completes successfully, the transaction can be saved to the database or "committed." A transaction typically involves several related commands, as in the case of a bank transfer. If Client A orders a transfer of funds to Client B, at least two database-access commands must be executed: § Client A's account must be debited. § Client B's account must be credited. If one of these commands is executed but the other is not, the funds will either vanish from Client A's account without appearing in Client B's account, or, perhaps worse from the viewpoint of the bank, the funds will be credited to Client B's account without being withdrawn from Client A's account, leaving the bank in the hole. This situation obviously becomes dramatically more complicated in the real world, where a large financial institution, with hundreds or thousands of users all accessing the database at the same time, can potentially have vast numbers of incomplete transactions active at any given moment. The solution is to combine logically related commands into groups that are committed as a single transaction. If a problem arises, the entire transaction can be rolled back and the problem fixed without serious adverse impact on business operations. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -204 - The primary commands used in managing transactions are COMMIT and R OLLBACK. As their names suggest, the COMMIT command commits changes made from the beginning of the transaction to the point at which the command is issued, and the ROLLBACK command undoes them. In addition, most databases support the AUTOCOMMIT option, which tells the RDBMS to commit all commands individually, as they are executed. This option can be used with the SET command. For example: SET AUTOCOMMIT [ON | OFF] ; By default, the SET AUTOCOMMIT ON command is executed at startup, telling the RDBMS to commit all statements automatically as they are executed. If you do not want these commands to be automatically executed, set the AUTOCOMMIT option to off as follows: SET AUTOCOMMIT OFF; When you start to work with a transaction, turn Autocommit off; then issue the commands required by the transaction, and, assuming that everything executes correctly, commit the transaction using this command: COMMIT; If any problems should arise during the transaction, you can cancel the entire transaction by using the following command: ROLLBACK; Note Transaction-management syntax varies considerably from one database management system to the next, but the basic syntax shown previously is supported by all common database management systems. The use of COMMIT and ROLLBACK in a JDBC example is very straightforward. Here's a modification to the example of Listing 6-3, which specifically turns Autocommit on. Simply insert the con.setAutoCommit(true) line into the stmt.execute(SQLCommand) method, as shown: public void execute(String SQLCommand){ String url = urlRoot+dbName; try { Connection con = DriverManager.getConnection(url); con.setAutoCommit(true); Statement stmt = con.createStatement(); stmt.execute(SQLCommand); con.close(); } catch(SQLException e){ System.err.print(e.getMessage()); } } Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -205- Adding the setAutoCommit(true) line tells the database management system to commit all changes automatically. If you compile and execute the modified code, you should get exactly the same results as you do when you run the original example. Now modify the code to turn Autocommit off, using setAutoCommit(false), as shown here: public void execute(String SQLCommand){ String url = urlRoot+dbName; try { Connection con = DriverManager.getConnection(url); con.setAutoCommit(false); Statement stmt = con.createStatement(); stmt.execute(SQLCommand); con.close(); } catch(SQLException e){ System.err.print(e.getMessage()); } } This time, when you run the example, it throws an "Invalid Transaction State" exception, and the update has not been made. The exception is thrown because we have not terminated the transaction before closing the connection. Now alter the code in the try block to the following; the change is made as before, because we have specifically told the database management system to commit the change: try { Connection con = DriverManager.getConnection(url); con.setAutoCommit(false); Statement stmt = con.createStatement(); stmt.execute(SQLCommand); con.commit(); con.close(); } If you change the try block by replacing the con.commit() with con.rollback(), the change will be rolled back, so no change will be visible. This time, however, no exception is thrown, as you can see here: try { Connection con = DriverManager.getConnection(url); Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -206 - con.setAutoCommit(false); Statement stmt = con.createStatement(); stmt.execute(SQLCommand); //con.commit(); con.rollback(); con.close(); } You can check to see if the UPDATE has been executed by inserting a SELECT statement to read the updated value of the Street field after the update command is executed but before it is rolled back. The try block now looks like this: try { Connection con = DriverManager.getConnection(url); con.setAutoCommit(false); Statement stmt = con.createStatement(); stmt.execute(SQLCommand); String query = "SELECT Street FROM Contact_Info "+ "WHERE First_Name = 'Michael' AND Last_Name ='Corleone';"; ResultSet rs = stmt.executeQuery(query); rs.next(); System.out.println("Street = "+rs.getString(1)); con.rollback(); con.close(); } When you run this version, it shows that the new value of Street matches the update, but when you look in the database, the previous value is still there because the change has been rolled back. Cross-Reference RecordSets and the SELECT are discussed in detail in Chapter 7 . The DELETE Statement The last data-manipulation command is DELETE, which is used for deleting entire records or groups of records. Again, when using the DELETE command, you use a WHERE clause to identify the records to be deleted. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -207- Use of the DELETE command is very straightforward. For example, this is the command you use to delete records containing the First_Name: "Michael" and the Last_Name: "Corleone": DELETE FROM Contact_Info WHERE First_Name = 'Michael' AND Last_Name = 'Corleone'; Caution INSERT, DELETE, and UPDATE can cause referential integrity problems with other tables, as well as significant problems within the table you are working on. Delete with care. A Swing-Based Table Editor To illustrate the topics covered in this chapter, the Swing-based table builder created in Chapter 5 is extended by the addition of a table editor (see Figure 6-1). The table editor is based on components derived from components built in Chapter 5. A new Edit menu (with Insert, Update, Delete, JMenuItems) and a new JTable in a JInternalFrame (for handling the Insert, Edit, and Delete functions) are also added. Figure 6-1: Inserting data with SQL INSERT The events are as follows: 1. The user selects a database. 2. The user selects an action: Insert, Update, or Delete. 3. The user selects the table. 4. A TableEdit frame is displayed for user interaction. 5. A SQL command is created dynamically and executed on command. The first step in building the table editor is to create the Edit menu by subclassing the DBMenu convenience class. The DBMenuItems Insert, Update, and Delete to the Edit menu are added and hooked into the Jframe, which forms the basis of the MainFrame class. Listing 6-4: Edit menu with insert, update, and delete items Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Chapter 6:Inserting, Updating, and Deleting Data -208- package jdbc_bible.part2; import java.awt.*; import java.awt.event.*; import java.util.Vector; import javax.swing.*; import javax.swing.event.*; public class EditMenu extends DBMenu{ JMenuItem insertItem; JMenuItem updateItem; JMenuItem deleteItem; JMenuItem exitItem; public EditMenu(){ setText("Edit"); setActionCommand("Edit"); setMnemonic((int)'E'); insertItem = new DBMenuItem("Insert",'I',itemListener,false); updateItem = new DBMenuItem("Update",'U',itemListener,false); deleteItem = new DBMenuItem("Delete",'D',itemListener,false); add(insertItem); add(updateItem); add(deleteItem); } } As discussed in Chapter 5, the DBMenu base class and the DBMenuItem class are simply convenience classes for building menus. Using these convenience classes simplifies the menu code considerably. TableEditFrame TableEditFrame, shown in Listing 6-5, is very similar to the TableBuilderFrame discussed in Chapter 5. It extends JInternalFrame and contains a JTable used to set up the fields for the Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... Deleting Data database table It also contains a JTextArea, which provides a preview of the generated SQL command, and an "Insert Data" button Listing 6-5: TableEditFrame package jdbc_bible.part2; import java. awt.*; import java. awt.event.*; import java. util.EventObject; import java. util.EventListener; import java. util.Vector; import javax.swing.*; import javax.swing.table.*; /** AM FL Y import javax.swing.event.*;... new JInternalFrame, TableEditFrame Listing 6-6: DatabaseManager — Controller class package jdbc_bible.part2; import java. awt.*; import java. awt.event.*; import java. util.Vector; import javax.swing.*; import javax.swing.event.*; public class DBManager extends JFrame{ JMenuBar menuBar = new JMenuBar(); JDesktopPane desktop = new JDesktopPane(); String database = null; String tableName = null; String... the DatabaseUtilities class shown in Listing 6-7, a second version of the execute() method has been added This new version accepts a String array argument so that it can loop through a number of SQL INSERT commands Listing 6-7: DatabaseUtilities — JDBC code package jdbc_bible.part2; import java. awt.event.*; import java. sql.*; import java. util.Vector; import sun.jdbc.odbc.JdbcOdbcDriver; public class DatabaseUtilities{... purchase PDF Split-Merge on www.verypdf.com to remove this watermark Chapter 6:Inserting, Updating, and Deleting Data private void selectDatabase(){ database = JOptionPane.showInputDialog(this, "Database: ","Select database" , JOptionPane.QUESTION_MESSAGE); dbUtils = new DatabaseUtilities(); dbUtils.setExceptionListener(new ExceptionListener()); tableMenu.enableMenuItem("New Table",true); tableMenu.enableMenuItem("Drop... of returning the table information required: § DatabaseMetaData, which returns information at the database level § ResultSetMetaData, which returns information at the ResultSet level The reason for using the ResultSetMetaData object in this example is to restrict the column information to just the columns being displayed and to defer discussion of the DatabaseMetaData object until ResultSets have been... Chapter 7 discusses retrieving data from a database by using the SELECT command -221 - Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Chapter 7:Retrieving Data with SQL Queries Chapter 7: Retrieving Data withSQL Queries In This Chapter One of the most important functions of any database application is finding the records in the database tables and returning them in the... void dropTable(){ tableName = JOptionPane.showInputDialog(this,"Table:", "Select table",JOptionPane.QUESTION_MESSAGE); int option = JOptionPane.showConfirmDialog(null, "Dropping table "+tableName, "Database " +database, JOptionPane.OK_CANCEL_OPTION); if(option==0){ executeSQLCommand("DROP TABLE "+tableName); } } class MenuListener implements ActionListener{ public void actionPerformed(ActionEvent event){... purchase PDF Split-Merge on www.verypdf.com to remove this watermark Chapter 6:Inserting, Updating, and Deleting Data String menuSelection = event.getActionCommand(); if(menuSelection.equals( "Database" )){ selectDatabase(); }else if(menuSelection.equals("New Table")){ displayTableBuilderFrame(); }else if(menuSelection.equals("Drop Table")){ dropTable(); }else if(menuSelection.equals("Insert")){ displayTableEditFrame();... protected DatabaseUtilities dbUtils; protected String tableName = null; protected String colNames[] = null; protected String dataTypes[] = null; protected String SQLCommand[] = null; protected String SQLCommandRoot = ""; -209 - Team-Fly® Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Chapter 6:Inserting, Updating, and Deleting Data public TableEditFrame(String tableName, DatabaseUtilities... added for Chapter 6 DatabaseUtilities dbUtils = null; TableMenu tableMenu = new TableMenu(); EditMenu editMenu = new EditMenu(); // added for Chapter 6 -213- Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Chapter 6:Inserting, Updating, and Deleting Data MenuListener menuListener = new MenuListener(); public DBManager(){ setJMenuBar(menuBar); setTitle("JDBC Database Bible"); . import java. awt.*; import java. awt.event.*; import java. util.EventObject; import java. util.EventListener; import java. util.Vector; import javax.swing.*;. jdbc_bible.part2; import java. awt.*; import java. awt.event.*; import java. util.Vector; import javax.swing.*; import javax.swing.event.*; public