Example of SQL-Injection C# SQL

OK. i want to know something about SQL injection.

I have a database with two table one is Logins and the other Orders.
i have a non-parametrized SQL Query like following.

// Select_Button click event 
//connection con
//command comm
comm.commandtext = "Select * from Logins where User_na='"+textBox1.text+" Pass_wrd='"+textBox2.text+"'";
//Execute reader

//Insrt_button Event
//connection ins_con
//command ins_comm
ins_comm.commandText = "Insert Into Logins(User_na, Pass_wrd) values ('"+textBox3.text+'", '"+textBox4.text+"'")";
//Execute non-query

Now i want to know how can there be SQL Attack on My database. How can i drop, say for example, my other datatables in the database.? Is it possible ?

Any and all help is highly appreciated.

Answer

Just to show how a Sql Injection is really easy and, apart from destruction of data, could lead to other nasty effects

textbox1.Text = "' OR User_na LIKE '%'; --";

the resulting comm.CommandText is

comm.commandtext = @"Select * from Logins where User_na='' OR User_na LIKE '%'--pass_wrd= 'xxx'";
SqlDataReader r = cmd.ExecuteReader();
if(r.HasRows)
{
    MessageBox.Show("The poor programmer was tricked by a smart hacker");
    .....
}

then depending on how do you check the results of the query the unauthenticated user could gain access to your program