Retrieve Operation using String Builder in C# Windows form Application

private void btnRetrieve_Click(object sender, EventArgs e)
{
    if (!string.IsNullOrEmpty(txtId.Text) || !string.IsNullOrEmpty(txtName.Text) || !string.IsNullOrEmpty(txtLocation.Text))
    {
        StringBuilder SB = new StringBuilder("select * from Employee where ");
        if (!string.IsNullOrEmpty(txtId.Text))
        {
            SB.Append("Emp_id='" + txtId.Text + "'");
        }
        if (!string.IsNullOrEmpty(txtName.Text))
        {
            if (!string.IsNullOrEmpty(txtId.Text))
            {
                SB.Append(" and ");
            }
            SB.Append("Emp_name='" + txtName.Text + "'");
        }
        if (!string.IsNullOrEmpty(txtLocation.Text))
        {
            if (!string.IsNullOrEmpty(txtId.Text) || !string.IsNullOrEmpty(txtName.Text))
            {
                SB.Append(" and ");
            }
            SB.Append("Emp_location='" + txtLocation.Text + "'");
        }
        SqlConnection con = new SqlConnection(constr);
        con.Open();
        SqlDataAdapter da = new SqlDataAdapter(SB.ToString(), con);
        DataSet ds = new DataSet();
        da.Fill(ds);
        dataGridView1.DataSource = ds.Tables[0];
        con.Close();
    }
    else
    {
        MessageBox.Show("Give atleast one data for search....!");
    }
}