Use Database in C#

I recently write a simple program that use database.

I've created an DB1.MDF and get data from it.

When I try to do Select from its Table (i.e. Table1), it gets run time error!

How I should do that?!

One more question:

I've created Data Grid View and put data in that. Now, how I can for example use column[2] of it (e.g. column[2] in my dataGridView is LastName and I want to save all LastNames in one String).

Here is my Code:

private void button1_Click(object sender, EventArgs e)
{

     SqlConnection conn = new SqlConnection();

     conn.ConnectionString = @"Data Source=.\SQLEXPRESS;
           AttachDbFilename=|DataDirectory|\DB1.mdf;
           Integrated Security=True;User Instance=True";

     conn.Open();

     SqlCommand cmd = new SqlCommand();

     cmd.Connection = conn;

     cmd.CommandText = "SELECT ID, Name, LastName from Table1 WHERE Name == Joe ";

     SqlDataAdapter da = new SqlDataAdapter();

     da.SelectCommand 开发者_如何学JAVA= cmd;

     DataTable dt = new DataTable();
     da.Fill(dt);

     dataGridView1.DataSource = dt;
     conn.Close();
 }

 private void button2_Click(object sender, EventArgs e)
 {
   textBox1.Text = dataGridView1.Columns[0].ToString();
 } 

The select query should be like this

cmd.CommandText = "SELECT ID, Name, LastName from Table1 WHERE Name = 'Joe'";

And for your second question, you could use something like this

private void button2_Click(object sender, EventArgs e)
{
     StringBuilder builder = new StringBuilder();
     dataGirdView1.Rows.Cast<DataGridViewRow>().ToList()
         .ForEach(r => builder.Append(r.Cells[2].Value));
     textBox1.Text = builder.ToString();
}

Bala R has it right, the == operator does not exist in SQL, therefore if your testing for an equality you should use the single = operator. And if you're comparing strings (varchar or nvarchar in SQL) you should put them in single quotes i.e. 'Joe'.

This query seems wrong.

SELECT ID, Name, LastName from Table1 WHERE Name == Joe 

Can you try this?

cmd.CommandText = "SELECT ID, Name, LastName from Table1 WHERE Name = 'Joe'"

Also, please post the details of the error message.