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    <p>LINQ does contain a way to group your data. Start by taking a look at this statement:</p> <pre><code>// group by Year-Month var rows = from s in saleList orderby s.MonthYear group s by new { Year = s.MonthYear.Year, Month = s.MonthYear.Month }; </code></pre> <p>The above statement will take your data and group it by Year-Month so that it will create a main key for each Year-Month combination and create a group of all corresponding <code>SaleMonth</code> items into that group.</p> <p>When you grasp that, the next step is to use those groups to calculate whatever you want to calculate within each group. So, if you were just looking to total all the <code>FullMonths</code> and <code>DaysMonths</code> for each Year-Month, you could do this:</p> <pre><code>var rowsTotals = from s in saleList orderby s.MonthYear group s by new { Year = s.MonthYear.Year, Month = s.MonthYear.Month } into grp select new { YearMonth = grp.Key.Year + " " + grp.Key.Month, FullMonthTotal = grp.Sum (x =&gt; x.FullMonth), DaysMonthTotal = grp.Sum (x =&gt; x.DaysMonth) }; </code></pre> <p><strong>Edit:</strong></p> <p>After looking again at what you're doing, I think it would be more efficient to do this:</p> <pre><code>// populate our class with the time period we are interested in var startDate = saleList.Min (x =&gt; x.ActivationDate); var endDate = saleList.Max (x =&gt; x.EndDate); List&lt;SaleMonth&gt; salesReport = new List&lt;SaleMonth&gt;(); for(var i = new DateTime(startDate.Year, startDate.Month, 1); i &lt;= new DateTime(endDate.Year, endDate.Month, 1); i = i.AddMonths(1)) { salesReport.Add(new SaleMonth { MonthYear = i }); } // loop through each Month-Year foreach(var sr in salesReport) { // get all the sales that happen in this month var lastDayThisMonth = sr.MonthYear.AddMonths(1).AddDays(-1); var sales = from s in saleList where lastDayThisMonth &gt;= s.ActivationDate, where sr.MonthYear &lt;= s.EndDate select s; // calculate the number of days the sale spans for just this month var nextMonth = sr.MonthYear.AddMonths(1); var firstOfNextMonth = sr.MonthYear.AddMonths(1).AddDays(-1).Day; sr.DaysMonth = sales.Sum (x =&gt; (x.EndDate &lt; nextMonth ? x.EndDate.Day : firstOfNextMonth) - (sr.MonthYear &gt; x.ActivationDate ? sr.MonthYear.Day : x.ActivationDate.Day)); // how many sales occur over the entire month sr.FullMonth = sales.Where (x =&gt; x.ActivationDate &lt;= sr.MonthYear &amp;&amp; nextMonth &lt; x.EndDate).Count (); } </code></pre>
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    1. COReally thank you! I like the way of filling dates and then go through them and take data accordingly. In addition to retrieving all sales by the date given is very helpful for me somewhere else - on clicking the "details" for each row in the report. So +1 twice =)
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    2. COHi Ijust get this error on the Sum func.. "only parameterless constructors and initializers are supported in linq to entities" i try to change the addMonth(1) to a external var - monthLetter but stil get this exception' do you have eny idle?
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    3. CO@oCcSking, yes, for Linq-to-Entities, you must remove all your parameterless constructors that are being created. If you take a look at my edit, I've changed all the queries that should get you going again and at least will get you running to make tweaks.
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