I need to list the trade dates, and stock closing prices for the each month of the year (which I will refer to as the 'current' month) and the stock closing price for the month preceding the 'current' month. 

The `StockData` table I am using has has columns for date, Stock high, low, opening, closing, and volume for each trade date and ticker symbol.

Initially, I used `AVG`:

```
SELECT 
    YEAR(TradeDate) AS Year, 
    MONTH(TradeDate) AS Month, 
    TickerSymbol, 
    AVG(ST_Close) AS CurrentClose,
    LAG (AVG(ST_Close), 1, 0) OVER (ORDER BY TickerSymbol, YEAR(TradeDate), MONTH(TradeDate), TickerSymbol) AS PreviousMonthClose,
FROM 
    StockData
WHERE
    TradeDate >= '2000-01-01' AND ST_Close IS NOT NULL
GROUP BY 
    YEAR(TradeDate), MONTH(TradeDate), TickerSymbol
ORDER BY 
    YEAR(TradeDate), MONTH(TradeDate), TickerSymbol;

```

But instead of using the average, I'd like to pull the stock closing price from the last day of each month, and the previous last day of the last month (for April 2010, I want to show the Stock Closing price on April 30th, 2010 and March 31st, 2010)

Any ideas on how to do this?