SQL command does not work

SqlCommand cmd1 = new SqlCommand("IF (Select UnitsInStock from Products WHERE BarCode=@BarCode) > @Quantity BEGIN UPDATE Products SET UnitsInStock=(UnitsInStock-@Quantity) WHERE BarCode=@BarCode END ELSE BEGIN PRINT 'There are not enough items in stock'END", new SqlConnection(Program.ConnectionString));
            cmd1.Connection.Open();
            for (int i = 0; i < dtbProducts.Rows.Count; i++)
            {
                cmd1.Parameters.AddWithValue("Quantity", (int)dtbProducts.Rows[i]["Quantity"]);
                cmd1.Parameters.AddWithValue("BarCode", dtbProducts.Rows[i]["BarCode"].ToString());
                cmd1.ExecuteNonQuery();
                cmd1.Parameters.Clear();
            }

            cmd1.Connection.Close();
            cmd.Connection.Close();
            this.Close();

        }