sql-server add tag
Paul White (imported from SE)
I often need to select a number of rows from each group in a result set.

For example, I might want to list the 'n' highest or lowest recent order values per customer.

In more complex cases, the number of rows to list might vary per group (defined by an attribute of the grouping/parent record). This part is definitely optional/for extra credit and not intended to dissuade people from answering.

What are the main options for solving these types of problems in SQL Server 2005 and later? What are the main advantages and disadvantages of each method?

**AdventureWorks examples (for clarity, optional)**

1. List the five most recent recent transaction dates and IDs from the `TransactionHistory` table, for each product that starts with a letter from M to R inclusive.
2. Same again, but with `n` history lines per product, where `n` is five times the `DaysToManufacture` Product attribute.
3. Same, for the special case where exactly one history line per product is required (the single most recent entry by `TransactionDate`, tie-break on `TransactionID`.
Top Answer
aaron bertrand (imported from SE)
The typical way to do this in SQL Server 2005 and up is to use a CTE and windowing functions. For top n per group you can simply use `ROW_NUMBER()` with a `PARTITION` clause, and filter against that in the outer query. So, for example, the top 5 most recent orders per customer could be displayed this way:

    DECLARE @top INT;
    SET @top = 5;
 
    ;WITH grp AS 
    (
       SELECT CustomerID, OrderID, OrderDate,
         rn = ROW_NUMBER() OVER
         (PARTITION BY CustomerID ORDER BY OrderDate DESC)
       FROM dbo.Orders
    )
    SELECT CustomerID, OrderID, OrderDate
      FROM grp
      WHERE rn <= @top
      ORDER BY CustomerID, OrderDate DESC;

You can also do this with `CROSS APPLY`:

    DECLARE @top INT;
    SET @top = 5;

    SELECT c.CustomerID, o.OrderID, o.OrderDate
    FROM dbo.Customers AS c
    CROSS APPLY 
    (
        SELECT TOP (@top) OrderID, OrderDate 
    	FROM dbo.Orders AS o
        WHERE CustomerID = c.CustomerID
    	ORDER BY OrderDate DESC
    ) AS o
    ORDER BY c.CustomerID, o.OrderDate DESC;

With the additional option Paul specified, say the Customers table has a column indicating how many rows to include per customer:

    ;WITH grp AS 
    (
       SELECT CustomerID, OrderID, OrderDate,
         rn = ROW_NUMBER() OVER
         (PARTITION BY CustomerID ORDER BY OrderDate DESC)
       FROM dbo.Orders
    )
    SELECT c.CustomerID, grp.OrderID, grp.OrderDate
      FROM grp 
      INNER JOIN dbo.Customers AS c
      ON grp.CustomerID = c.CustomerID
      AND grp.rn <= c.Number_of_Recent_Orders_to_Show
      ORDER BY c.CustomerID, grp.OrderDate DESC;

And again, using `CROSS APPLY` and incorporating the added option that the number of rows for a customer be dictated by some column in the customers table:

    SELECT c.CustomerID, o.OrderID, o.OrderDate
    FROM dbo.Customers AS c
    CROSS APPLY 
    (
        SELECT TOP (c.Number_of_Recent_Orders_to_Show) OrderID, OrderDate 
    	FROM dbo.Orders AS o
        WHERE CustomerID = c.CustomerID
    	ORDER BY OrderDate DESC
    ) AS o
    ORDER BY c.CustomerID, o.OrderDate DESC;

Note that these will perform differently depending on data distribution and the availability of supporting indexes, so optimizing the performance and getting the best plan will really depend on local factors.

Personally, I prefer the CTE and windowing solutions over the `CROSS APPLY` / `TOP` because they separate the logic better and are more intuitive (to me). In general (both in this case and in my general experience), the CTE approach produces more efficient plans (examples below), but this should not be taken as a universal truth - you should always test your scenarios, especially if indexes have changed or data has skewed significantly.

----

## AdventureWorks examples - without any changes

> 1. List the five most recent recent transaction dates and IDs from the `TransactionHistory` table, for each product that starts with a letter from M to R inclusive.

    -- CTE / OVER()

    ;WITH History AS
    (
      SELECT p.ProductID, p.Name, t.TransactionID, t.TransactionDate,
        rn = ROW_NUMBER() OVER 
        (PARTITION BY t.ProductID ORDER BY t.TransactionDate DESC)
      FROM Production.Product AS p
      INNER JOIN Production.TransactionHistory AS t
      ON p.ProductID = t.ProductID
      WHERE p.Name >= N'M' AND p.Name < N'S'
    )
    SELECT ProductID, Name, TransactionID, TransactionDate
    FROM History 
    WHERE rn <= 5;

    -- CROSS APPLY
    
    SELECT p.ProductID, p.Name, t.TransactionID, t.TransactionDate
    FROM Production.Product AS p
    CROSS APPLY
    (
      SELECT TOP (5) TransactionID, TransactionDate
      FROM Production.TransactionHistory
      WHERE ProductID = p.ProductID
      ORDER BY TransactionDate DESC
    ) AS t
    WHERE p.Name >= N'M' AND p.Name < N'S';

Comparison of these two in runtime metrics:

![enter image description here][1]

CTE / `OVER()` plan:

![enter image description here][2]

`CROSS APPLY` plan:

![enter image description here][3]

The CTE plan looks more complicated, but it's actually much more efficient. Pay little attention to the estimated cost % numbers, but focus on more important *actual* observations, such as far fewer reads and a much lower duration. I also ran these without parallelism, and this wasn't the difference. Runtime metrics and the CTE plan (the `CROSS APPLY` plan remained the same):

![enter image description here][4]

![enter image description here][5]

> 2. Same again, but with `n` history lines per product, where `n` is five times the `DaysToManufacture` Product attribute.

Very minor changes required here. For the CTE, we can add a column to the inner query, and filter on the outer query; for the `CROSS APPLY`, we can perform the calculation inside the correlated `TOP`. You'd think this would lend some efficiency to the `CROSS APPLY` solution, but that doesn't happen in this case. Queries:

    -- CTE / OVER()

    ;WITH History AS
    (
      SELECT p.ProductID, p.Name, p.DaysToManufacture, t.TransactionID, t.TransactionDate,
        rn = ROW_NUMBER() OVER 
        (PARTITION BY t.ProductID ORDER BY t.TransactionDate DESC)
      FROM Production.Product AS p
      INNER JOIN Production.TransactionHistory AS t
      ON p.ProductID = t.ProductID
      WHERE p.Name >= N'M' AND p.Name < N'S'
    )
    SELECT ProductID, Name, TransactionID, TransactionDate
    FROM History 
    WHERE rn <= (5 * DaysToManufacture);

    -- CROSS APPLY
    
    SELECT p.ProductID, p.Name, t.TransactionID, t.TransactionDate
    FROM Production.Product AS p
    CROSS APPLY
    (
      SELECT TOP (5 * p.DaysToManufacture) TransactionID, TransactionDate
      FROM Production.TransactionHistory
      WHERE ProductID = p.ProductID
      ORDER BY TransactionDate DESC
    ) AS t
    WHERE p.Name >= N'M' AND p.Name < N'S';

Runtime results:

![enter image description here][6]

Parallel CTE / `OVER()` plan:

![enter image description here][7]

Single-threaded CTE / `OVER()` plan:

![enter image description here][8]

`CROSS APPLY` plan:

![enter image description here][9]

> 3. Same, for the special case where exactly one history line per product is required (the single most recent entry by `TransactionDate`, tie-break on `TransactionID`.

Again, minor changes here. In the CTE solution, we add `TransactionID` to the `OVER()` clause, and change the outer filter to `rn = 1`. For the `CROSS APPLY`, we change the `TOP` to `TOP (1)`, and add `TransactionID` to the inner `ORDER BY`.

    -- CTE / OVER()

    ;WITH History AS
    (
      SELECT p.ProductID, p.Name, t.TransactionID, t.TransactionDate,
        rn = ROW_NUMBER() OVER 
        (PARTITION BY t.ProductID ORDER BY t.TransactionDate DESC, TransactionID DESC)
      FROM Production.Product AS p
      INNER JOIN Production.TransactionHistory AS t
      ON p.ProductID = t.ProductID
      WHERE p.Name >= N'M' AND p.Name < N'S'
    )
    SELECT ProductID, Name, TransactionID, TransactionDate
    FROM History 
    WHERE rn = 1;
    
    -- CROSS APPLY

    SELECT p.ProductID, p.Name, t.TransactionID, t.TransactionDate
    FROM Production.Product AS p
    CROSS APPLY
    (
      SELECT TOP (1) TransactionID, TransactionDate
      FROM Production.TransactionHistory
      WHERE ProductID = p.ProductID
      ORDER BY TransactionDate DESC, TransactionID DESC
    ) AS t
    WHERE p.Name >= N'M' AND p.Name < N'S';

Runtime results:

![enter image description here][10]

Parallel CTE / `OVER()` plan:

![enter image description here][11]

Single-threaded CTE / OVER() plan:

![enter image description here][12]

`CROSS APPLY` plan:

![enter image description here][13]

Windowing functions aren't always the best alternative (have a go at `COUNT(*) OVER()`), and these are not the only two approaches to solving the n rows per group problem, but in this specific case - given the schema, existing indexes, and data distribution - the CTE fared better by all meaningful accounts.

----

## AdventureWorks examples - with flexibility to add indexes

However, if you add a supporting index, similar to [the one Paul mentioned in a comment](https://dba.stackexchange.com/questions/86415/retrieving-n-rows-per-group/86416#comment155628_86416) but with the 2nd and 3rd columns ordered `DESC`:

    CREATE UNIQUE NONCLUSTERED INDEX UQ3 ON Production.TransactionHistory 
      (ProductID, TransactionDate DESC, TransactionID DESC);

You would actually get much more favorable plans all around, and the metrics would flip to favor the `CROSS APPLY` approach in all three cases:

![enter image description here][14]

If this were my production environment, I'd probably be satisfied with the duration in this case, and wouldn't bother to optimize further.

----
*This was all much uglier in SQL Server 2000, which didn't support `APPLY` or the `OVER()` clause.*


  [1]: http://i.stack.imgur.com/iOXqU.png
  [2]: http://i.stack.imgur.com/KU3A8.png
  [3]: http://i.stack.imgur.com/UQdCH.png
  [4]: http://i.stack.imgur.com/IvBMZ.png
  [5]: http://i.stack.imgur.com/Uj0hq.png
  [6]: http://i.stack.imgur.com/3ttv7.png
  [7]: http://i.stack.imgur.com/W7bbs.png
  [8]: http://i.stack.imgur.com/lTgFD.png
  [9]: http://i.stack.imgur.com/BnBka.png
  [10]: http://i.stack.imgur.com/2lEnF.png
  [11]: http://i.stack.imgur.com/X6Gyn.png
  [12]: http://i.stack.imgur.com/0wDW5.png
  [13]: http://i.stack.imgur.com/9UEip.png
  [14]: http://i.stack.imgur.com/3tw5C.png
Answer #2
rob farley (imported from SE)
Let's start with the basic scenario.

If I want to get some number of rows out of a table, I have two main options: ranking functions; or `TOP`.

First, let's consider the whole set from `Production.TransactionHistory` for a particular `ProductID`:

    SELECT h.TransactionID, h.ProductID, h.TransactionDate
    FROM Production.TransactionHistory h
    WHERE h.ProductID = 800;

This returns 418 rows, and the plan shows that it checks every row in the table looking for this - an unrestricted Clustered Index Scan, with a Predicate to provide the filter. 797 reads here, which is ugly.

![Expensive Scan with 'Residual' Predicate][1]

So let's be fair to it, and create an index that would be more useful. Our conditions call for an equality match on `ProductID`, followed by a search for the most recent by `TransactionDate`. We need the `TransactionID` returned too, so let's go with: `CREATE INDEX ix_FindingMostRecent ON Production.TransactionHistory (ProductID, TransactionDate) INCLUDE (TransactionID);`.

Having done this, our plan changes significantly, and drops the reads down to just 3. So we're already improving things by over 250x or so...

![Improved plan][2]

Now that we've levelled the playing field, let's look at the top options - ranking functions and `TOP`.

    WITH Numbered AS
    (
    SELECT h.TransactionID, h.ProductID, h.TransactionDate, ROW_NUMBER() OVER (ORDER BY TransactionDate DESC) AS RowNum
    FROM Production.TransactionHistory h
    WHERE h.ProductID = 800
    )
    SELECT TransactionID, ProductID, TransactionDate
    FROM Numbered
    WHERE RowNum <= 5;
    
    SELECT TOP (5) h.TransactionID, h.ProductID, h.TransactionDate
    FROM Production.TransactionHistory h
    WHERE h.ProductID = 800
    ORDER BY TransactionDate DESC;

![Two plans - basic TOP\RowNum][3]

You will notice that the second (`TOP`) query is much simpler than the first, both in query and in plan. But very significantly, they both use `TOP` to limit the number of rows actually being pulled out of the index. The costs are only estimates and worth ignoring, but you can see a lot of similarity in the two plans, with the `ROW_NUMBER()` version doing a tiny amount of extra work to assign numbers and filter accordingly, and both queries end up doing just 2 reads to do their work. The Query Optimizer certainly recognises the idea of filtering on a `ROW_NUMBER()` field, realising that it can use a Top operator to ignore rows that aren't going to be needed. Both these queries are good enough - `TOP` isn't so much better that it's worth changing code, but it is simpler and probably clearer for beginners.

So this work across a single product. But we need to consider what happens if we need to do this across multiple products.

The iterative programmer is going to consider the idea of looping through the products of interest, and calling this query multiple times, and we can actually get away with writing a query in this form - not using cursors, but using `APPLY`. I'm using `OUTER APPLY`, figuring that we might want to return the Product with NULL, if there are no Transactions for it.

    SELECT p.Name, p.ProductID, t.TransactionID, t.TransactionDate
    FROM 
    Production.Product p
    OUTER APPLY (
        SELECT TOP (5) h.TransactionID, h.ProductID, h.TransactionDate
        FROM Production.TransactionHistory h
        WHERE h.ProductID = p.ProductID
        ORDER BY TransactionDate DESC
    ) t
    WHERE p.Name >= 'M' AND p.Name < 'S';

The plan for this is the iterative programmers' method - Nested Loop, doing a Top operation and Seek (those 2 reads we had before) for each Product. This gives 4 reads against Product, and 360 against TransactionHistory.

![APPLY plan][4]

Using `ROW_NUMBER()`, the method is to use `PARTITION BY` in the `OVER` clause, so that we restart the numbering for each Product. This can then be filtered like before. The plan ends up being quite different. The logical reads are about 15% lower on TransactionHistory, with a full Index Scan going on to get the rows out.

    WITH Numbered AS
    (
    SELECT p.Name, p.ProductID, h.TransactionID, h.TransactionDate, ROW_NUMBER() OVER (PARTITION BY h.ProductID ORDER BY h.TransactionDate DESC) AS RowNum
    FROM Production.Product p
    LEFT JOIN Production.TransactionHistory h ON h.ProductID = p.ProductID
    WHERE p.Name >= 'M' AND p.Name < 'S'
    )
    SELECT Name, ProductID, TransactionID, TransactionDate
    FROM Numbered n
    WHERE RowNum <= 5;

![ROW_NUMBER plan][5]

Significantly, though, this plan has an expensive Sort operator. The Merge Join doesn't seem to maintain the order of rows in TransactionHistory, the data must be resorted to be able to find the rownumbers. It's fewer reads, but this blocking Sort could feel painful. Using `APPLY`, the Nested Loop will return the first rows very quickly, after just a few reads, but with a Sort, `ROW_NUMBER()` will only return rows after a most of the work has been finished.

Interestingly, if the `ROW_NUMBER()` query uses `INNER JOIN` instead of `LEFT JOIN`, then a different plan comes up.

![ROW_NUMBER() with INNER JOIN][6]

This plan uses a Nested Loop, just like with `APPLY`. But there's no Top operator, so it pulls all the transactions for each product, and uses a lot more reads than before - 492 reads against TransactionHistory. There isn't a good reason for it not to choose the Merge Join option here, so I guess the plan was considered 'Good Enough'. Still - it doesn't block, which is nice - just not as nice as `APPLY`.

The `PARTITION BY` column that I used for `ROW_NUMBER()` was `h.ProductID` in both cases, because I had wanted to give the QO the option of producing the RowNum value before joining to the Product table. If I use `p.ProductID`, we see the same shape plan as with the `INNER JOIN` variation.

    WITH Numbered AS
    (
    SELECT p.Name, p.ProductID, h.TransactionID, h.TransactionDate, ROW_NUMBER() OVER (PARTITION BY p.ProductID ORDER BY h.TransactionDate DESC) AS RowNum
    FROM Production.Product p
    LEFT JOIN Production.TransactionHistory h ON h.ProductID = p.ProductID
    WHERE p.Name >= 'M' AND p.Name < 'S'
    )
    SELECT Name, ProductID, TransactionID, TransactionDate
    FROM Numbered n
    WHERE RowNum <= 5;

But the Join operator says 'Left Outer Join' instead of 'Inner Join'. The number of reads is still just under 500 reads against the TransactionHistory table.

![PARTITION BY on p.ProductID instead of h.ProductID][7]

Anyway - back to the question at hand...

We've answered **question 1**, with two options that you could pick and choose from. Personally, I like the `APPLY` option.

To extend this to use a variable number (**question 2**), the `5` just needs to be changed accordingly. Oh, and I added another index, so that there was an index on `Production.Product.Name` that included the `DaysToManufacture` column.

    WITH Numbered AS
    (
    SELECT p.Name, p.ProductID, p.DaysToManufacture, h.TransactionID, h.TransactionDate, ROW_NUMBER() OVER (PARTITION BY h.ProductID ORDER BY h.TransactionDate DESC) AS RowNum
    FROM Production.Product p
    LEFT JOIN Production.TransactionHistory h ON h.ProductID = p.ProductID
    WHERE p.Name >= 'M' AND p.Name < 'S'
    )
    SELECT Name, ProductID, TransactionID, TransactionDate
    FROM Numbered n
    WHERE RowNum <= 5 * DaysToManufacture;
    
    SELECT p.Name, p.ProductID, t.TransactionID, t.TransactionDate
    FROM 
    Production.Product p
    OUTER APPLY (
        SELECT TOP (5 * p.DaysToManufacture) h.TransactionID, h.ProductID, h.TransactionDate
        FROM Production.TransactionHistory h
        WHERE h.ProductID = p.ProductID
        ORDER BY TransactionDate DESC
    ) t
    WHERE p.Name >= 'M' AND p.Name < 'S';

And both plans are almost identical to what they were before!

![Variable rows][8]

Again, ignore the estimated costs - but I still like the TOP scenario, as it is so much more simple, and the plan has no blocking operator. The reads are less on TransactionHistory because of the high number of zeroes in `DaysToManufacture`, but in real life, I doubt we'd be picking that column. ;)

One way to avoid the block is to come up with a plan that handles the `ROW_NUMBER()` bit to the right (in the plan) of the join. We can persuade this to happen by doing the join outside the CTE.

    WITH Numbered AS
    (
    SELECT h.TransactionID, h.ProductID, h.TransactionDate, ROW_NUMBER() OVER (PARTITION BY ProductID ORDER BY TransactionDate DESC) AS RowNum
    FROM Production.TransactionHistory h
    )
    SELECT p.Name, p.ProductID, t.TransactionID, t.TransactionDate
    FROM Production.Product p
    LEFT JOIN Numbered t ON t.ProductID = p.ProductID
        AND t.RowNum <= 5 * p.DaysToManufacture
    WHERE p.Name >= 'M' AND p.Name < 'S';

The plan here looks simpler - it's not blocking, but there's a hidden danger.

![Joining outside CTE][9]

Notice the Compute Scalar that's pulling data from the Product table. This is working out the `5 * p.DaysToManufacture` value. This value isn't being passed into the branch that's pulling data from the TransactionHistory table, it's being used in the Merge Join. As a Residual.

![Sneaky Residual!][11]

So the Merge Join is consuming ALL the rows, not just the first however-many-are-needed, but all of them and then doing a residual check. This is dangerous as the number of transactions increases. I'm not a fan of this scenario - residual predicates in Merge Joins can quickly escalate. Another reason why I prefer the `APPLY/TOP` scenario.

In the special case where it's exactly one row, for **question 3**, we can obviously use the same queries, but with `1` instead of `5`. But then we have an extra option, which is to use regular aggregates.

    SELECT ProductID, MAX(TransactionDate)
    FROM Production.TransactionHistory
    GROUP BY ProductID;

A query like this would be a useful start, and we could easily modify it to pull out the TransactionID as well for tie-break purposes (using a concatenation which would then be broken down), but we either look at the whole index, or we dive in product by product, and we don't really get a big improvement on what we had before in this scenario.

But I should point out that we're looking at a particular scenario here. With real data, and with an indexing strategy that may not be ideal, mileage may vary considerably. Despite the fact that we've seen that `APPLY` is strong here, it can be slower in some situations. It rarely blocks though, as it has a tendency to use Nested Loops, which many people (myself included) find very appealing.

I haven't tried to explore parallelism here, or dived very hard into question 3, which I see as a special case that people rarely want based on the complication of concatenating and splitting. The main thing to consider here is that these two options are both very strong.

I prefer `APPLY`. It's clear, it uses the Top operator well, and it rarely causes blocking.


  [1]: https://i.stack.imgur.com/NjxAr.png
  [2]: https://i.stack.imgur.com/SuSo4.png
  [3]: https://i.stack.imgur.com/9O7zx.png
  [4]: https://i.stack.imgur.com/jXxSl.png
  [5]: https://i.stack.imgur.com/ipmb0.png
  [6]: https://i.stack.imgur.com/rBft7.png
  [7]: https://i.stack.imgur.com/bLMqE.png
  [8]: https://i.stack.imgur.com/NIgfd.png
  [9]: https://i.stack.imgur.com/KeQyu.png
  [10]: https://i.stack.imgur.com/u830P.png
  [11]: https://i.stack.imgur.com/rSBwi.png
Answer #3
ypercubeᵀᴹ (imported from SE)
In DBMS, like MySQL, that do not have window functions or `CROSS APPLY`, the way to do this would be to use standard SQL (89). The slow way would be a triangular cross join with aggregate. The faster way (but still and probably not as efficient as using cross apply or the row_number function) would be what I call the ***"poor man's `CROSS APPLY`"***. It would be interesting to compare this query with the others:

*Assumption: `Orders (CustomerID, OrderDate)` has a `UNIQUE` constraint:*

    DECLARE @top INT;
    SET @top = 5;
 
    SELECT o.CustomerID, o.OrderID, o.OrderDate
      FROM dbo.Customers AS c
        JOIN dbo.Orders AS o
          ON  o.CustomerID = c.CustomerID
          AND o.OrderID IN
              ( SELECT TOP (@top) oi.OrderID
                FROM dbo.Orders AS oi
                WHERE oi.CustomerID = c.CustomerID
                ORDER BY oi.OrderDate DESC
              )
      ORDER BY CustomerID, OrderDate DESC ;

For the extra problem of customized top rows per group:

    SELECT o.CustomerID, o.OrderID, o.OrderDate
      FROM dbo.Customers AS c
        JOIN dbo.Orders AS o
          ON  o.CustomerID = c.CustomerID
          AND o.OrderID IN
              ( SELECT TOP (c.Number_of_Recent_Orders_to_Show) oi.OrderID
                FROM dbo.Orders AS oi
                WHERE oi.CustomerID = c.CustomerID
                ORDER BY oi.OrderDate DESC
              )
      ORDER BY CustomerID, OrderDate DESC ;

*Note: In MySQL, instead of `AND o.OrderID IN (SELECT TOP(@top) oi.OrderID ...)` one would use `AND o.OrderDate >= (SELECT oi.OrderDate ... LIMIT 1 OFFSET (@top - 1))`. SQL-Server added `FETCH / OFFSET` syntax in 2012 version. The queries here were adjusted with `IN (TOP...)` to work with earlier versions.*
Answer #4
mikael eriksson (imported from SE)
`APPLY TOP` or `ROW_NUMBER()`? What could there possibly be more to say on that matter?

A short recap of the differences and to really keep it short I will only show the plans for option 2 and I have added the index on `Production.TransactionHistory`.

    create index IX_TransactionHistoryX on 
      Production.TransactionHistory(ProductID, TransactionDate)

 The `row_number()` query:.

    with C as
    (
      select T.TransactionID,
             T.TransactionDate,
             P.DaysToManufacture,
             row_number() over(partition by P.ProductID order by T.TransactionDate desc) as rn
      from Production.Product as P
        inner join Production.TransactionHistory as T
          on P.ProductID = T.ProductID
      where P.Name >= N'M' and
            P.Name < N'S'
    )
    select C.TransactionID,
           C.TransactionDate
    from C
    where C.rn <= 5 * C.DaysToManufacture;

![enter image description here][1]

The `apply top` version:

    select T.TransactionID, 
           T.TransactionDate
    from Production.Product as P
      cross apply (
                  select top(cast(5 * P.DaysToManufacture as bigint))
                    T.TransactionID,
                    T.TransactionDate
                  from Production.TransactionHistory as T
                  where P.ProductID = T.ProductID
                  order by T.TransactionDate desc
                  ) as T
    where P.Name >= N'M' and
          P.Name < N'S';


![enter image description here][2]


The main difference between these are that `apply top` filters on the top expression below the nested loops join where `row_number` version filters after the join. That means there are more reads from `Production.TransactionHistory` than really is necessary.

If there only existed a way to push the operators responsible for enumerating rows down to the lower branch before the join then `row_number` version might do better.

So enter `apply row_number()` version.


    select T.TransactionID, 
           T.TransactionDate
    from Production.Product as P
      cross apply (
                  select T.TransactionID,
                         T.TransactionDate
                  from (
                       select T.TransactionID,
                              T.TransactionDate,
                              row_number() over(order by T.TransactionDate desc) as rn
                       from Production.TransactionHistory as T
                       where P.ProductID = T.ProductID
                       ) as T
                  where T.rn <= cast(5 * P.DaysToManufacture as bigint)
                  ) as T
    where P.Name >= N'M' and
          P.Name < N'S';

![enter image description here][3]


As you can see `apply row_number()` is pretty much the same as `apply top` only slightly more complicated. Execution time is also about the same or bit slower. 

So why did I bother to come up with an answer that is no better than what we already have? Well, you have one more thing to try out in the real world and there actually is a difference in reads. One that I don't have an explanation for[^1].

    APPLY - ROW_NUMBER
    (961 row(s) affected)
    Table 'TransactionHistory'. Scan count 115, logical reads 230, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
    Table 'Product'. Scan count 1, logical reads 15, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
    
    APPLY - TOP
    (961 row(s) affected)
    Table 'TransactionHistory'. Scan count 115, logical reads 268, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
    Table 'Product'. Scan count 1, logical reads 15, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.



While I'm at it i might as well throw in a second `row_number()` version that in certain cases might be the way to go. Those certain cases would be when you expect you actually need most of the rows from `Production.TransactionHistory` because here you get a merge join between `Production.Product` and the enumerated `Production.TransactionHistory`.

    with C as
    (
      select T.TransactionID,
             T.TransactionDate,
    		 T.ProductID,
             row_number() over(partition by T.ProductID order by T.TransactionDate desc) as rn
      from Production.TransactionHistory as T
    )
    select C.TransactionID,
           C.TransactionDate
    from C
     inner join Production.Product as P
          on P.ProductID = C.ProductID
    where P.Name >= N'M' and
          P.Name < N'S' and
          C.rn <= 5 * P.DaysToManufacture;

![enter image description here][4]


To get the above shape without a sort operator you also have to change the supporting index to order by `TransactionDate` descending.

    create index IX_TransactionHistoryX on 
      Production.TransactionHistory(ProductID, TransactionDate desc)

---

[^1]: The extra logical reads are due to the [nested loops prefetching][5] used with the apply-top. You can disable this with undoc'd TF 8744 (and/or 9115 on later versions) to get the same number of logical reads. Prefetching could be an advantage of the apply-top alternative in the right circumstances.


  [1]: https://i.stack.imgur.com/Bdkw4.png
  [2]: https://i.stack.imgur.com/ixKBI.png
  [3]: https://i.stack.imgur.com/agcVw.png
  [4]: https://i.stack.imgur.com/hCi6Z.png
  [5]: https://sql.kiwi/2013/08/sql-server-internals-nested-loops-prefetching.html

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