def calculate_sma(data, period):
    """
    Calculate the Simple Moving Average (SMA) for the given data.
    :param data: A list of dictionaries containing 'date' and 'close' prices.
    :param period: period used to calculate sma.
    :return: A list of dictionaries with an additional 'SMA' key.
    """
    close_prices = [entry['close'] for entry in data]
    sma_values = []

    for i in range(len(close_prices)):
        if i + 1 < period:
            sma_values.append(None)
        else:
            sma = sum(close_prices[i + 1 - period:i + 1]) / period
            sma_values.append(sma)

    for i in range(len(data)):
        data[i]['SMA'] = sma_values[i]

    return data