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C HAPTE R 11 CONN ECT IVIT Y AN D NET WORK IN G
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To relay your SQL statements to the database, you need to create a JDBC Statement object. This object will associate itself with an open connection and henceforth act as the conduit through which SQL statements are transferred from the Java program to the database for execution. Here s how you create the JDBC Statement object: statement stmt = conn.createStatement(); No SQL statements are associated with the stmt object. However, under the Statement class, there is another object called PreparedStatement that always contains a SQL statement in addition to being the channel for the statement s execution. This SQL statement is compiled immediately, and it can be compiled just once and used many times thereafter, which is a great benefit. For simplicity, however, I ll just use the Statement object in this discussion. Let s now turn to the execution of SQL statements.
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You can understand JDBC SQL statements if you separate the SELECT statements that query the database from all the other statements. Unlike the others, SELECT statements don t change the state of the database. Let s first look at how to deal with query statements.
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Handling Queries
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SELECT statements use the executeQuery() method to get the query results. The method returns the results in the ResultSet object. Listing 11-8 shows an example. Listing 11-8. Getting the Query Results string first_name,last_name,manager; number salary; resultSet rs = stmt.executeQuery("SELECT * FROM Employees"); while (rs.next()) { first_name = rs.getString("first_name"); last_name = rs.getString("last_name"); manager = rs.getString("manager"); salary = rs.getNumber("salary"); system.out.println(first_name + last_name "works for" Manager "salary is:" salary."); Note that rs is an instance of the resultSet object, and it holds the query results. The resultSet object also provides a cursor, so you can access the results one by one. Each time you invoke the resultSet method, the cursor moves to the next row in the result set.
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Any statement that changes the state of the database be it a DDL statement or a DML statement such as INSERT, UPDATE, or DELETE is executed using the executeUpdate() method. Note that the word update in the method name indicates that the SQL statement will change something in the database. Here are some examples of executeUpdate() statements: statement stmt = conn.createStatement(); stmt.executeUpdate("CREATE TABLE Employees" +
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CHAPTER 11 C ON NEC TIVITY A ND NETWORKING
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"(last_name VARCHAR2(30), first_name VARCHAR2(20), manager VARCHAR2(30), salary(number"); stmt.executeUpdate("INSERT INTO Employees " + "VALUES ('Alapati', 'Valerie', 'Shannon', salary)"); The preceding statements create the Statement object, and then they create a table and insert some data into it. All your normal SQL transaction properties, such as consistency and durability, are maintained when you use JDBC to execute SQL statements. By default, each statement commits after its execution because the value of conn.setAutoCommit() is set to true, as you can see in the following example. You can ensure that there is a commit after every statement in either of the following ways (and if you wish, you can also use the conn.rollback() method to roll back a statement): conn.setAutoCommit(false); or conn.commit(); Here s a simple example that shows how to use the commit() and rollback() statements: conn.setAutoCommit(false); Statement stmt = conn.createStatement(); stmt.executeUpdate("INSERT INTO employees VALUES('Alapati','Valerie','Nicholas',50000 )"); conn.rollback(); stmt.executeUpdate("INSERT INTO employees VALUES('Alapati','Nina','Nicholas',50000)"); conn.commit();
Error Handling
All programs must have an exception handler built in; especially those DML statements that change the database state. One way to do this is to use the rollback() statement when you run into an error, so your partial changes are all undone. You can use the SQLException() method to catch errors. In Java programs, you use a try code block to generate (or throw) an exception, and the catch block will catch the exception thus thrown. Listing 11-9 shows a sample Java code block that illustrates these concepts. Listing 11-9. Handling Errors in Java try { conn.setAutoCommit(false); stmt.executeUpdate(" " + "(Disney World', 'MickeyMouse', 2.00)"); conn.commit(); conn.setAutoCommit(true); } catch(SQLException ex) { system.err.println("SQLException: " + ex.getMessage()); conn.rollback(); conn.setAutoCommit(true); }
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