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Try connOpen() result = commExecuteNonQuery() ConsoleWriteLine("Inserted {0} records", result) Catch e as NpgsqlException ConsoleWriteLine("Problem: {0}", eMessage) Finally connClose() End Try End Sub End Class
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The updatevb program declares the necessary variables using the Dim statement, then creates a connection to the local PostgreSQL server using the earl Login Role from 12 The CommandText string is built on three separate lines by concatenating a string value You can also place the command string directly in the CommandText property of the NpgsqlCommand constructor The SQL command is not executed until the ExecuteNonQuery() method is executed in the program This is done in a try-catch block to catch any possible errors without blowing up the program The ExecuteNonQuery() method returns the number of records affected by the command This value is displayed on the screen to verify that the INSERT command worked Compiling and running the example should look like this:
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C:\>vbc /r:npgsqldll /r:SystemDatadll updatevb Microsoft (R) Visual Basic Compiler version 805072742 for Microsoft (R) NET Framework version 205072742 Copyright (c) Microsoft Corporation All rights reserved C:\>update Inserted 1 records C:\>
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The compile finished successfully, and the program indicated that the record was added with no problems You can use pgAdmin III or psql to view the Bidder table to see if the record was indeed added You can also try to run the update program again and see what happens:
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C:\>update Problem: ERROR: 23505: duplicate key violates unique constraint "bidder_pkey" C:\>
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This time an NpgsqlException exception was thrown The exception indicates that the new record could not be added because it would violate the primary key constraint, which is what you would expect when trying to insert the same record again
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The ExecuteScalar() Method
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The ExecuteScalar() method is used to execute a command that you know will return a single value, such as the output of a built-in function or stored procedure Since the SQL command executed can return any type of data type, the ExecuteScalar() method returns a generic object data type, which must be typecast into the expected data type The getvalscs program demonstrates using the ExecuteScalar() method in a C# application:
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using System; using SystemData; using Npgsql; public class GetVals { public static void Main(String[] args) { String connstring = "Server=127001;Port=5432;SSL=True;"; connstring = connstring + "User ID=earl;Password=auction;Database=test;"; NpgsqlConnection conn = new NpgsqlConnection(connstring); NpgsqlCommand comm = new NpgsqlCommand(); commConnection = conn; commCommandText = "Select timeofday();"; try { connOpen(); String servertime = (String)commExecuteScalar(); ConsoleWriteLine("The time on the server is: {0}", servertime); commCommandText = "Select pi();"; Double result = (Double)commExecuteScalar(); ConsoleWriteLine("The value of pi on the server is: {0}", result); } catch( NpgsqlException e) { ConsoleWriteLine("problem: {0}", eMessage); } finally { connClose(); } } }
By now you should be familiar with the NpgsqlConnection and NpgsqlCommand parts Within the try-catch block, after the connection to the PostgreSQL server is opened, the ExecuteScalar() method is used to execute the first SQL command The return value is typecast into a String data type, and displayed
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Microsoft NET Framework
Following that, the NpgsqlCommand CommandText property is changed to a new SQL command, and the ExecuteScalar() method is used again, this time typecast to a Double data type This demonstrates that you can use the same NpgsqlCommand instance to send multiple SQL commands Compile and run the getvalscs program as normal, and watch the output:
C:\>csc /r:npgsqldll getvalscs Microsoft (R) Visual C# NET Compiler version 8005072742 for Microsoft (R) NET Framework version 2050727 Copyright (C) Microsoft Corporation 2001-2005 All rights reserved C:\>getvals The time on the server is: Mon Sep 18 19:20:01343000 2006 EDT The value of pi on the server is: 314159265358979 C:\>
The ExecuteReader() Method
Obviously, the type of SQL commands most often used in applications are SELECT commands, which query the database tables for data The result of the SELECT command is most often a result set that returns more than one record of data The key to being able to handle this situation in your NET program is the ExecuteReader() method The ExecuteReader() method reads the result set returned by the PostgreSQL server, and returns the results in an NpgsqlDataReader object Each record in the result set is contained in the NpgsqlDataReader object You must use the NpgsqlDataReader object to cycle through the records returned in the result set The NpgsqlDataReader object is an array value The array is automatically created and sized based on the returned result set Each value in the array is a column in the returned record, with zero being the first column returned in the result set, one being the second column in the result set, and so on until the last column Each record in the returned result set is read individually using the NpgsqlDataReader object After a record s values are read, you can advance to the next record in the result set You are only able to advance forward through the result set using the NpgsqlDataReader object You cannot go back to reread previous records The format for obtaining the result set from the NpgsqlCommand query looks like this:
NpgsqlDataReader data = commExecuteReader();
The NpgsqlDataReader class contains several properties that can be used to analyze the result set returned These are listed and described in Table 13-5 Besides these properties, there are also several class methods available to use in the NpgsqlDataReader class, listed and described in Table 13-6 The columns in each record in the result set are referenced by their index value You can extract the record column values into variables using the Getxxx methods You must know the column data type in the table, and extract the column data into the proper variable data type Any mismatches are marked as errors when the program runs
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