Return a null value as placeholder for full column
<p>I am joining Table1 to itself with a <code>LEFT JOIN</code> on columns <code>year</code>, <code>month</code>, and <code>market</code>. </p>

<pre><code> SELECT Source.Month
   ,Source.Year
   ,Source.Price
   ,Data.Price
FROM Table1 As Source
  LEFT JOIN Table1 As Data ON Source.Month = Data.Month
                           AND Source.Year = Data.Year
WHERE Source.Product = 1
  AND Data.Product = 2
</code></pre>

<p>The target value that I am attempting to populate, <code>Price</code>, exists for all months and years for the <code>Product</code> in <code>Source</code>, but only selected months and years for the <code>Product</code> in <code>Data</code>. I am trying to get every <code>year</code> and <code>month</code> to popuate regardless if <code>Data.Price</code> exists. I thought it would return <code>NULL</code> due to the <code>LEFT JOIN</code>, but I think it may have to do with the <code>WHERE</code> clause condition. </p>

<p>How can I get <code>Data.Price</code> to populate <code>NULL</code> in the case that no value for <code>Price</code> exists for the selected <code>Product</code>?</p>