SQL Querries and Tuto

Creating a SqlCommand Object

Similar to other C# objects, you instantiate a SqlCommand object via the new instance declaration, as follows:
    SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);
The line above is typical for instantiating a SqlCommand object.  It takes a string parameter that holds the command you want to execute and a reference to a SqlConnection object.  SqlCommand has a few overloads, which you will see in the examples of this tutorial.

Querying Data

When using a SQL select command, you retrieve a data set for viewing.  To accomplish this with a SqlCommand object, you would use the ExecuteReader method, which returns a SqlDataReader object.  We'll discuss the SqlDataReader in a future lesson.  The example below shows how to use the SqlCommand object to obtain a SqlDataReader object:
// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);

// 2. Call Execute reader to get query results
SqlDataReader rdr = cmd.ExecuteReader();
In the example above, we instantiate a SqlCommand object, passing the command string and connection object to the constructor.  Then we obtain a SqlDataReader object by calling the ExecuteReader method of the SqlCommand object, cmd. 
This code is part of the ReadData method of Listing 1 in the Putting it All Together section later in this lesson.

Inserting Data

To insert data into a database, use the ExecuteNonQuery method of the SqlCommand object.  The following code shows how to insert data into a database table:
// prepare command string
 string insertString = @"
     insert into Categories
     (CategoryName, Description)
     values ('Miscellaneous', 'Whatever doesn''t fit elsewhere')";

 // 1. Instantiate a new command with a query and connection
 SqlCommand cmd = new SqlCommand(insertString, conn);

 // 2. Call ExecuteNonQuery to send command
 cmd.ExecuteNonQuery();
The SqlCommand instantiation is just a little different from what you've seen before, but it is basically the same.  Instead of a literal string as the first parameter of the SqlCommand constructor, we are using a variable, insertString.  The insertString variable is declared just above the SqlCommand declaration. 
Notice the two apostrophes ('') in the insertString text for the word "doesn''t".  This is how you escape the apostrophe to get the string to populate column properly. 
Another observation to make about the insert command is that we explicitly specified the columns CategoryName and Description.  The Categories table has a primary key field named CategoryID.  We left this out of the list because SQL Server will add this field itself.  trying to add a value to a primary key field, such as CategoryID, will generate an exception.
To execute this command, we simply call the ExecuteNonQuery method on the SqlCommand instance, cmd.
This code is part of the Insertdata method of Listing 1 in the Putting it All Together section later in this lesson.

Updating Data

The ExecuteNonQuery method is also used for updating data.  The following code shows how to update data:
// prepare command string
 string updateString = @"
     update Categories
     set CategoryName = 'Other'
     where CategoryName = 'Miscellaneous'";
 
 // 1. Instantiate a new command with command text only
 SqlCommand cmd = new SqlCommand(updateString);
 
 // 2. Set the Connection property
 cmd.Connection = conn;
 
 // 3. Call ExecuteNonQuery to send command
 cmd.ExecuteNonQuery();
Again, we put the SQL command into a string variable, but this time we used a different SqlCommand constructor that takes only the command.  In step 2, we assign the SqlConnection object, conn, to the Connection property of the SqlCommand object, cmd. 
This could have been done with the same constructor used for the insert command, with two parameters.  It demonstrates that you can change the connection object assigned to a command at any time.
The ExecuteNonQuery method performs the update command.
This code is part of the UpdateData method of Listing 1 in the Putting it All Together section later in this lesson.

Deleting Data

You can also delete data using the ExecuteNonQuery method.  The following example shows how to delete a record from a database with the ExecuteNonQuery method:
// prepare command string
 string deleteString = @"
     delete from Categories
     where CategoryName = 'Other'";
 
 // 1. Instantiate a new command
 SqlCommand cmd = new SqlCommand();
 
 // 2. Set the CommandText property
 cmd.CommandText = deleteString;
 
 // 3. Set the Connection property
 cmd.Connection = conn;
 
 // 4. Call ExecuteNonQuery to send command
 cmd.ExecuteNonQuery();
This example uses the SqlCommand constructor with no parameters.  Instead, it explicity sets the CommandText and Connection properties of the SqlCommand object, cmd. 
We could have also used either of the two previous SqlCommand constructor overloads, used for the insert or update command, with the same result.  This demonstrates that you can change both the command text and the connection object at any time. 
The ExecuteNonQuery method call sends the command to the database.
This code is part of the DeleteData method of Listing 1 in the Putting it All Together section later in this lesson.

Getting Single values

Sometimes all you need from a database is a single value, which could be a count, sum, average, or other aggregated value from a data set.  Performing an ExecuteReader and calculating the result in your code is not the most efficient way to do this.  The best choice is to let the database perform the work and return just the single value you need.  The following example shows how to do this with the ExecuteScalar method:
// 1. Instantiate a new command
 SqlCommand cmd = new SqlCommand("select count(*) from Categories", conn);
 
 // 2. Call ExecuteNonQuery to send command
 int count = (int)cmd.ExecuteScalar();
The query in the SqlCommand constructor obtains the count of all records from the Categories table.  This query will only return a single value.  The ExecuteScalar method in step 2 returns this value.  Since the return type of ExecuteScalar is type object, we use a cast operator to convert the value to int.
This code is part of the GetNumberOfRecords method of Listing 1 in the Putting it All Together section later in this lesson.

Putting it All Together

For simplicity, we showed snippets of code in previous sections to demonstrate the applicable techniques .  It is also useful to have an entire code listing to see how this code is used in a working program.  Listing 1 shows all of the code used in this example, along with a driver in the Main method to produce formatted output.

Listing 1.  SqlConnection Demo

 using System;
 using System.Data;
 using System.Data.SqlClient;

 /// <summary>
 /// Demonstrates how to work with SqlCommand objects
 /// </summary>
 class SqlCommandDemo
 {
     SqlConnection conn;
 
     public SqlCommandDemo()
     {
         // Instantiate the connection
         conn = new SqlConnection(
            "Data Source=(local);Initial Catalog=Northwind;Integrated Security=SSPI");
     }
 
     // call methods that demo SqlCommand capabilities
     static void Main()
     {
         SqlCommandDemo scd = new SqlCommandDemo();

         Console.WriteLine();
         Console.WriteLine("Categories Before Insert");
         Console.WriteLine("------------------------");
 
         // use ExecuteReader method
         scd.ReadData();
 
         // use ExecuteNonQuery method for Insert
         scd.Insertdata();
         Console.WriteLine();
         Console.WriteLine("Categories After Insert");
         Console.WriteLine("------------------------------");

        scd.ReadData();

         // use ExecuteNonQuery method for Update
         scd.UpdateData();

         Console.WriteLine();
         Console.WriteLine("Categories After Update");
         Console.WriteLine("------------------------------");

         scd.ReadData();

         // use ExecuteNonQuery method for Delete
         scd.DeleteData();

         Console.WriteLine();
         Console.WriteLine("Categories After Delete");
         Console.WriteLine("------------------------------");

         scd.ReadData();

         // use ExecuteScalar method
         int numberOfRecords = scd.GetNumberOfRecords();

         Console.WriteLine();
         Console.WriteLine("Number of Records: {0}", numberOfRecords);
     }

     /// <summary>
     /// use ExecuteReader method
     /// </summary>
     public void ReadData()
     {
        SqlDataReader rdr = null;
 
         try
         {
             // Open the connection
             conn.Open();
 
             // 1. Instantiate a new command with a query and connection
             SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);
 
             // 2. Call Execute reader to get query results
             rdr = cmd.ExecuteReader();

             // print the CategoryName of each record
             while (rdr.Read())
             {
                 Console.WriteLine(rdr[0]);
             }
         }
         finally
         {
             // close the reader
             if (rdr != null)
             {
                 rdr.Close();
             }
 
             // Close the connection
             if (conn != null)
             {
                 conn.Close();
             }
         }
     }

     /// <summary>
     /// use ExecuteNonQuery method for Insert
     /// </summary>
     public void Insertdata()
     {
         try
         {
             // Open the connection
             conn.Open();
 
             // prepare command string
             string insertString = @"
                 insert into Categories
                 (CategoryName, Description)
                 values ('Miscellaneous', 'Whatever doesn''t fit elsewhere')";
 
             // 1. Instantiate a new command with a query and connection
             SqlCommand cmd = new SqlCommand(insertString, conn);
 
             // 2. Call ExecuteNonQuery to send command
             cmd.ExecuteNonQuery();
         }
         finally
         {
             // Close the connection
             if (conn != null)
             {
                 conn.Close();
             }
         }
     }
 
     /// <summary>
     /// use ExecuteNonQuery method for Update
     /// </summary>
     public void UpdateData()
     {
         try
         {
             // Open the connection
             conn.Open();
 
             // prepare command string
             string updateString = @"
                 update Categories
                 set CategoryName = 'Other'
                 where CategoryName = 'Miscellaneous'";
 
             // 1. Instantiate a new command with command text only
             SqlCommand cmd = new SqlCommand(updateString);

             // 2. Set the Connection property
             cmd.Connection = conn;
 
             // 3. Call ExecuteNonQuery to send command
             cmd.ExecuteNonQuery();
        }
         finally
         {
             // Close the connection
             if (conn != null)
             {
                 conn.Close();
             }
         }
     }
 
     /// <summary>
     /// use ExecuteNonQuery method for Delete
     /// </summary>
     public void DeleteData()
     {
         try
         {
             // Open the connection
             conn.Open();
 
             // prepare command string
             string deleteString = @"
                 delete from Categories
                 where CategoryName = 'Other'";
 
             // 1. Instantiate a new command
             SqlCommand cmd = new SqlCommand();
 
             // 2. Set the CommandText property
             cmd.CommandText = deleteString;
 
             // 3. Set the Connection property
             cmd.Connection = conn;
 
             // 4. Call ExecuteNonQuery to send command
             cmd.ExecuteNonQuery();
         }
         finally
         {
             // Close the connection
             if (conn != null)
             {
                 conn.Close();
             }
         }
     }

     /// <summary>
     /// use ExecuteScalar method
     /// </summary>
     /// <returns>number of records</returns>
     public int GetNumberOfRecords()
     {
         int count = -1;
 
         try
         {
             // Open the connection
             conn.Open();
 
             // 1. Instantiate a new command
             SqlCommand cmd = new SqlCommand("select count(*) from Categories", conn);
 
             // 2. Call ExecuteScalar to send command
             count = (int)cmd.ExecuteScalar();
         }
         finally
         {
            // Close the connection
             if (conn != null)
             {
                 conn.Close();
             }
         }
         return count;
     }
 }
In Listing 1, the SqlConnection object is instantiated in the SqlCommandDemo structure.  This is okay because the object itself will be cleaned up when the CLR garbage collector executes.  What is important is that we close the connection when we are done using it.  This program opens the connection in a try block and closes it in a finally block in each method.
The ReadData method displays the contents of the CategoryName column of the Categories table.  We use it several times in the Main method to show the current status of the Categories table, which changes after each of the insert, update, and delete commands.  Because of this, it is convenient to reuse to show you the effects after each method call.

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