When we have a more complex query that retrieves records from different tables, or don't want the UPDATE command to check for changes in the database before updating it, we can specify our own SQL commands.The Command object enables  to execute queries against  data source. in order to retrieve data, you must know the schema of your database as well as how to build a valid SQL query. The Command objects allow developers to specify parameters dynamically at run time. When defining the SQL statement we use a placeholder instead of a particular value. Then we use the Parameters collection of the Command object to define the dynamic column value.The SqlCommand object must be used in conjunction with the SqlConnection object

SqlCommand cmd = new SqlCommand("select CategoryName from Categories", new sqlconnection(parameter));

Querying Data

using a SQL select command

1)Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand("select EmployeeName from Emp", conn);

 2. Call Execute reader to get query results
SqlDataReader dr = cmd.ExecuteReader();

Inserting Data

using a SQL insert command,To insert data into a database, use the ExecuteNonQuery method of the SqlCommand object.

1)Insert command string
 string insertString = @"
     insert into Emp
     (EmpName, Post)
     values ('Aditya', 'S\w Engg')";
 
 2.)Instantiate a new command with a query and connection
 SqlCommand cmd = new SqlCommand(insertString, conn);
 
 3) Call ExecuteNonQuery to send command
 cmd.ExecuteNonQuery();

Updating Data

using a SQL update command,To update data into a database, use the ExecuteNonQuery method of the SqlCommand object.

Deleting Data

using a SQL delete command,To delete data into a database, use the ExecuteNonQuery method of the SqlCommand object.

Getting Single Values