Querying with Window Functions

In this lesson, you will learn how to leverage SQL window functions within Entity Framework Core by using raw SQL projections to perform advanced calculations like running totals or rankings.

Why use Window Functions?

While EF Core’s LINQ provider is excellent for standard filtering and grouping, it doesn't natively translate complex window functions (like RANK(), LEAD(), or SUM() OVER()) into SQL. When you need to calculate a value based on a set of rows related to the current row without collapsing the result set into a single group, raw SQL projections are your best tool.

Implementing with FromSql

To use these features, you define a DTO (Data Transfer Object) to hold the results of the window function, then map the raw SQL query to that type.

Imagine you have a Sales table and want to calculate a running total of sales per salesperson:

public class SalesReportDto
{
    public string Salesperson { get; set; }
    public decimal Amount { get; set; }
    public decimal RunningTotal { get; set; }
}

// Using the context to execute the window function
var report = context.Database
    .SqlQuery<SalesReportDto>($@"
        SELECT 
            Salesperson, 
            Amount, 
            SUM(Amount) OVER (PARTITION BY Salesperson ORDER BY Date) as RunningTotal
        FROM Sales")
    .ToList();

The Power of Projections

By using SqlQuery<T>, you bypass the standard entity tracking and mapping. This is highly efficient for reporting scenarios. Because you are using raw SQL, you gain access to the full power of your database engine’s windowing capabilities, such as ROW_NUMBER() for pagination or LAG() to compare a current record's value with the previous one.

Remember that because this returns an IQueryable, you can still chain standard LINQ methods like .Where() or .OrderBy() after the initial SQL projection if necessary, though it is usually cleaner to include those filters directly in the SQL string to ensure the database handles the workload.

Key Takeaway: Use SqlQuery<T> to project results from raw SQL window functions when your business logic requires row-specific calculations that standard LINQ cannot express.