Sending SQL Commands in VS .NET

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Sending SQL Commands
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SQL commands are contained in a Statement class object They are created using the createStatement() method from the Connection object:
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Connection conn = DriverManagergetConnection(url, user, passwd); Statement stmt = conncreateStatement();
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Once the Statement class object is created, you can use it to send SQL commands to the server There are two methods that are used to do this: executeUpdate() executeQuery()
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Figure 15-8 Creating the Connectjava program using NetBeans
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The executeUpdate() method is used to send nonqueries, such as INSERT, DELETE, and UPDATE SQL commands, to the server The executeQuery() method is used to send queries using the SELECT SQL command to the server The following sections demonstrate both of these methods
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Processing Nonqueries
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Performing SQL commands that do not return a result set (such as INSERT, DELETE, and UPDATE) requires using the executeUpdate() method of the Statement class The executeUpdate() method has one parameter, the SQL command as a String value, and returns an integer value indicating the number of records affected by the command:
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int result = stmtexecuteUpdate("INSERT into auctionbidder values(
If an error occurs in the SQL command processing, the executeUpdate() method throws a Java Exception, which should be caught by the program
PostgreSQL 8 for Windows
The Updatejava program demonstrates inserting data using the executeUpdate() method:
import javasql*; public class Update { public static void main(String[] args) { String URL = "jdbc:postgresql://127001/test"; String user = "earl"; String passwd = "auction"; Connection sqlcon = null; try { ClassforName("orgpostgresqlDriver"); sqlcon = DriverManagergetConnection(URL, user, passwd); Systemoutprintln("connected to database"); Statement stmt = sqlconcreateStatement(); String command = "INSERT into auctionbidder values "; command = command + "(126, 'Pierce', 'Alex', NULL,"; command = command + " NULL, NULL, NULL, '555-5392')"; int result = stmtexecuteUpdate(command); Systemoutprintln("added " + result + " record"); stmtclose(); sqlconclose(); } catch (Exception e) { Systemoutprintln("Problem: " + egetMessage()); } } }
The Updatejava program uses the standard JDBC methods to load the driver and connect to the database Next, a Statement object is created and the executeUpdate() method is used with a normal String value that contains a SQL INSERT command Compiling and running the program produces the following results:
C:\test> javac Updatejava C:\test> java -classpath C:\test;C:\test\postgresql-81-405jdbc3jar Update connected to database added 1 records C:\test> java -classpath C:\test;C:\test\postgresql-81-405jdbc3jar Update connected to database Problem: ERROR: duplicate key violates unique constraint "bidder_pkey" C:\test>
The first time the Updatejava program is run, the record is added successfully You can use the standard pgAdmin III program to check the inserted value If you try
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Java
to run the program again to perform the same INSERT command, a Java exception will be thrown The error message produced specifies the cause of the exception As expected, the new record was rejected because the primary key value already existed in the table
Processing Queries
Performing SQL commands that return either a single data value (such as functions) or a multirecord result set (such as SELECT queries) requires that you use the executeQuery() method of the Statement class In both situations, the method returns data in a ResultSet object:
Statement stmt = sqlconcreateStatement(); ResultSet rset = stmtexecuteQuery("SELECT * from auctionbidder");
The ResultSet object contains the result set data retrieved from the executed SQL command You can move back and forth between records in the result set using the next() method to go forward and the previous() method to go backward in the ResultSet object You can also read a specific record in the result set using the absolute() method One oddity of the ResultSet object is that when it returns with result set data, it does not point to the first record in the result set, even if there is only one record in the result set Instead it points to just before the first record When the result set returns, you must perform a next() method to point to the first record in the result set This is demonstrated in the GetValsjava program:
import javasql*; public class GetVals { public static void main(String[] args) { String URL = "jdbc:postgresql://127001/test"; String user = "earl"; String passwd = "auction"; Connection sqlcon = null; Statement stmt = null; ResultSet rset = null; try { ClassforName("orgpostgresqlDriver"); sqlcon = DriverManagergetConnection(URL, user, passwd); Systemoutprintln("connected to database"); stmt = sqlconcreateStatement(); rset = stmtexecuteQuery("SELECT timeofday()"); rsetnext(); String day = rsetgetString(1); Systemoutprintln("time: " + day);
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