Equivalent of VLOOKUP in DAX – Part II – using TOPN

In my previous entry, I presented the LOOKUPVALUE function. In this article, I will explain how you can use the TOPN function to mimic the behavior of VLOOKUP when doing “approximate” searches.

Ken Puls’s article presents a typical use case for this pattern. Let me start with briefly sketching his problem and describe how he solves it.

The effective tax rate problem

Ken uses data representing sales at a daily level. His problem is to determine the effective tax rate for a given row, when tax rates change over time.

In his article, he uses meaningful sales data. Since the core of the problem is to find the relevant tax rate for a specific date, and I want to focus on that, my ‘Data’ table looks like this:

Date
06/01/2010
07/01/2010
08/01/2010
03/01/2013
04/01/2013
05/01/2013

Here is the ‘Tax Rates’ table. [Start Date] represents the day when a tax rate became effective.

Start Date Tax Rate
07/01/2010 0,12
04/01/2013 0,05

Note that Gerhard Brueckl’s problem is similar: he presents a way to assign the relevant fiscal period to a date in a dimension table. In his model, fiscal periods are defined by their start date as well.

A solution

Ken Puls’ idea is to add a calculated column to his data. This column will contain the value of [Start Date] for the relevant tax rate. He will then create a relationship between the ‘Data’ table and ‘Tax Rates’ table based on this new column.

His calculated column contains the MAX value of [Start Date] for all rows of ‘Tax Rate’ where ‘Tax Rate’[Start Date] is not greater than the current date in the data table. In other words, he replicates the logic of the following Excel formula:

= VLOOKUP( [Date] , 'Tax Rates', 1, TRUE)

The DAX expression for a calculated column looks like this:

[Tax Rate Start Date] := 
 CALCULATE(  MAX( 'Tax Rates'[Start Date]) 
   , FILTER('Tax Rates', 'Tax Rates'[Start Date] <= 'Data'[Date] )  
 )

It filters the ‘Tax Rates’ table, and calculates the max value of ‘Tax Rates'[Start Date] for the filtered table.

Extending the solution (TOPN)

We can use TOPN to mimic the behaviour of VLOOKUP.

For an approximate search, we will follow this logic:

  • use a FILTER expression to eliminate irrelevant rows (as in the previous formula)
  • use the TOPN function to filter the result and only return the last row in ‘Tax Rates’
  • return the value in the required column for the row we just found.

Here, “the last row”  means the row with the latest [Start Date].

Let us rewrite Ken’s expression using the TOPN function:

[Effective Tax Rate - Start Date] :=
 CALCULATE (  VALUES('Tax Rates'[Start Date])
  , TOPN(1
  , FILTER('Tax Rates', 'Tax Rates'[Start Date] <= 'Data'[Date]  )
   , 'Tax Rates'[Start Date]
    )
 )

The advantage of this pattern is that we can use it to query any column in our lookup table:

[Effective Tax Rate - Rate] :=
 CALCULATE ( VALUES('Tax Rates'[Tax Rate])
  , TOPN(1
   , FILTER('Tax Rates', 'Tax Rates'[Start Date] <= 'Data'[Date]  )
   , 'Tax Rates'[Start Date]
    )
 )

Dealing with ties

There might be a problem with the previous formula: TOPN may return more than 1 row. This will be the case, if there are ties in your data.

In our example, if the ‘Tax Rates’ table has several entries for the same [Start Date], then the VALUES expression might return more than one value. In that case, the expression will fail.

Examples:

Start Date
Tax Rate
Start Date
Tax Rate
07/01/2010 0,12 07/01/2010 0,12
04/01/2013 0,05

04/01/2013 0,05

07/01/2010 0,12 07/01/2010 0,13
The formula still works. The formula returns an error.

This may be totally fine. Most of the time, this is what we want: if a reference table contains contradictory information, we want to notice it.

However, in some cases, you may want your formula to ignore that, and take one of these values anyway.

In that case, you can add a sort expression to the TOPN expression, so as to guarantee that only one row (or value) is returned.

Edit: Although this is what I first wrote below, SAMPLE does not behave like a TOPN with ties. How I came to this conclusion is unclear, as I can no longer find the test data I used. Although, this might seem to be true in this specific case, you should not assume this will work every time.

However, you can wrap the TOPN expression within a SAMPLE expression to only get one row (or more if required).

You may also use the SAMPLE function instead of the TOPN function. What does it do? Basically the same as TOPN except in case of a tie. In that case, SAMPLE will exactly return the required number of rows.

  • Which rows? The engine will decide by itself.
  • Will it always return the same rows? This is not guaranteed.
  • Will the rows returned change on each execution? This is not guaranteed.

As a side-note, if you want to write a query that returns exactly 3 rows from your ‘Data’ table, but want these rows to be randomly drawn between each execution, then you can do the following.

EVALUATE SAMPLE( 3,'Data', RAND() )

Equivalent of VLOOKUP in DAX–Part I

One frequent DAX requirement is to write a formula that behaves like the VLOOKUP function in Excel.

Gehrard Brueckl recently blogged about how to map a date to a fiscal period, when fiscal periods are defined in a separate table, and each period is defined by its start date. Ken Puls recently wrote an article about calculating an effective tax rate with DAX.

In this first post, I will present the LOOKUPVALUE function.

Quick notes about VLOOKUP

VLOOKUP, a classic Excel function, accepts two values (TRUE or FALSE) for its last argument.

Assuming we defined names for Value, Range, i, the formula will look like this:
=VLOOKUP(Value, Range, i, FALSE)

VLOOKUP will look for the first occurrence of Value in the first column of Range. If the value cannot be found, the formula will return #NA, else it will return the value in the i-th column on the same row.

Note that VLOOKUP also works with arrays.

When the last parameter is TRUE (or omitted), then VLOOKUP requires your data to be sorted in ascending order. It will look for the last value in the first column of Range that is not greater than Value, and will return the value in the i-th column on the same row, or #NA if none exists.

More details on VLOOKUP here, there, or everywhere on the internet.

LOOKUPVALUE

Syntax

DAX V2 introduced a LOOKUPVALUE, that has a related behavior.

A notable difference is that LOOKUPVALUE allows you to provide criteria on several columns. In Excel formulas, this would require you to use concatenate columns, or even replace VLOOKUP with some INDEX/MATCH formula.

The syntax looks like this:
= LOOKUPVALUE( Table[OutputColumn], Table[LookupColumn1], “SearchValue1”,  Table[LookupColumn1],  “SearchValue2”)

The syntax looks like the vector form of the LOOKUP function, if you put the order of arguments aside. It actually looks like the syntax of the LOOKUPS function, which does not actually exist. Smile

Basically, you first have to define which column contains your output, then define your first lookup column, then provide the value you are looking for, then define your second lookup column, and so on …

Behavior

LOOKUPVALUE behaves differently from VLOOKUP:

  1. whereas VLOOKUP works with arrays, LOOKUPVALUE does not work with column expressions. In other words, the columns arguments must reference columns that physically exist in your model. Also, remember that each column must belong to the same table.
  2. if several rows in your table match your criteria, then VLOOKUP in Excel will only return the first (or last) match. On the contrary, LOOKUPVALUE in DAX may return an error if several rows match your criteria. This will occur if [OutputColumn] does not contain the same value for all matching rows.
  3. a BLANK value is returned if there is no match.

Also, not surprisingly, you must address columns by names and cannot use indexes to do this.

When to use LOOKUP value?

When your lookup table is a parameter of your model. By that, I mean a static table that you can import in your model.

Obviously, when the behavior described above suits your requirements.

When you want to calculate search values on the fly, or use different search values that come from different tables. In other words, when you cannot use PowerPivot relationships.

Bug in PowerPivot 2012 for Excel 2010

I spotted what I firmly believe to be a bug in PowerPivot. If you already met this problem and/or can reproduce this issue, please support this ticket on MS Connect.

Edit: This bug was fixed with SQL Server 2012 Service Pack 2.

An MS employee might – someday , maybe – respond to my bug report and say this is “by design”. This employee may also advance some pretty good reasons why this is so. However, until then, I cannot conceive of any good reason for this behavior.

Introduction

The data

For the rest of this post, I will use an embedded PowerPivot model based on a linked table. The table is called ‘SomeData’:

Country Product OtherAttribute
USA A Y
FRA B Y
USA C N
FRA C N
UK A N
GER B N
USA A N
FRA B N
FRA D N

What is this bug?

Instead of an elaborate discussion about this bug, let me just start introducing it with a mini-quiz.

Questions

Question 1:

Our model only contains the table ‘SomeData’ presented above. We run the following query against our model.

EVALUATE
CALCULATETABLE(     VALUES(‘SomeData’) ,
SUMMARIZE(
FILTER( ‘SomeData’, [OtherAttribute] = “Y”)
, [Country], [Product]
)
)

What does the query return?

  1. All rows for which Country = “USA” and Product = “A” or Country = “FRA” and Product = “B”.
  2. All rows for which Country = “USA” or Country = “FRA”
  3. All rows for which Product = “A” or Product = “B”.
  4. It depends

Question 2:

Our model only contains the table ‘SomeData’ presented above. We run the following query against our model.

EVALUATE
CALCULATETABLE( ‘SomeData’,
SUMMARIZE(
FILTER( ‘SomeData’, [OtherAttribute] = “Y”)
, [Country], [Product]
)
)

What does the query return?

  1. All rows for which Country = “USA” and Product = “A” or Country = “FRA” and Product = “B”.
  2. All rows for which Country = “USA” or Country = “FRA”
  3. All rows for which Product = “A” or Product = “B”.
  4. It depends

Answers:

Question 1:

1. All rows for which Country = “USA” and Product = “A” or Country = “FRA” and Product = “B”.

Question 2:

4. It depends: The result can be either 2 or 3.

Stating the problem

The only difference between both queries, is the presence (or absence) of the VALUES function in the first argument. This means the expression passed as a first argument to the CALCULATE statement changes the way the 2nd argument is evaluated. This is only supposed to work the other way around.

In the second case, the evaluation of the context is incorrect. PowerPivot will only filter on one column. Which one depends on the order of the columns in the ‘SomeData’ Excel table.

As a result, a query might  have different results depending on how the workbook was built.

A working example (direct filtering) …

Returning an arbitrary set of tuples

Consider the following query:

EVALUATE
SUMMARIZE(
FILTER( ‘SomeData’
, [Country] = “USA” && [Product] = “A”
|| [Country] = “FRA” && [Product] = “B”
)
, [Country], [Product]
)

This query returns the set of (Country, Product) tuples according to my predicates:

SomeData[Country] SomeData[Product]
USA A
FRA B

Filtering data based on a calculated set

If I want to get all rows from ‘SomeData’, where the tuple (Product,  Country) matches one of the tuples in the previous set, I can use the previous table expression as the 2nd argument of a CALCULATETABLE expression. This gives the following query:

EVALUATE
CALCULATETABLE( ‘SomeData’,
SUMMARIZE(
FILTER( ‘SomeData’
, [Country] = “USA” && [Product] = “A”
|| [Country] = “FRA” && [Product] = “B”
)
, [Country], [Product]
)
)

The results:

SomeData[Country] SomeData[Product] SomeData[OtherAttribute]
USA A Y
FRA B Y
USA A N
FRA B N

So far so good.  Everything works as expected.

… that no longer works (cross-filtering)

Returning an arbitrary set of tuples

Instead of explicitly filtering the (Country, Product) tuples, as above, let us now filter our data on the [OtherAttribute] column.

EVALUATE
SUMMARIZE(
FILTER( ‘SomeData’, [OtherAttribute] = “Y”)
, [Country], [Product]
)

As before, this returns:

SomeData[Country] SomeData[Product]
USA A
FRA B

Filtering data based on a calculated set

Let us plug, our new SUMMARIZE expression into our CALCULATETABLE expression.

EVALUATE
CALCULATETABLE( ‘SomeData’,
SUMMARIZE(
FILTER( ‘SomeData’, [OtherAttribute] = “Y”)
, [Country], [Product]
)
)

Unexpectedly, this time, the query returns the following results:

SomeData[Country] SomeData[Product] SomeData[OtherAttribute]
USA A Y
FRA B Y
UK A N
GER B N
USA A N
FRA B N

Note that the engine now returns all rows from ‘SomeData’ for which [Product] is in { “A”, “B” }. This is confirmed by SQL Server Profiler.

Replacing ‘SomeData’ with VALUES(‘SomeData’), however, will return the expected results.

Reordering the columns

Worse, let us just change the order of the columns in our Excel table.

Just drag and drop the [Country] column to the left of the [Product] column, and refresh the PowerPivot model. We have not made any changes to the PowerPivot model itself.

Let us run our previous query again.

The results now become:

Country Product OtherAttribute
USA A Y
FRA B Y
USA C N
FRA C N
USA A N
FRA B N
FRA D N

Which means, the query now only filters by Country. Once again, this is confirmed by SQL Server Profiler.

When a filter context is available

Finally, let us add another CALCULATETABLE expression, so that our query becomes:

EVALUATE
CALCULATETABLE(
CALCULATETABLE( ‘SomeData’,
SUMMARIZE(
FILTER( (‘SomeData’), [OtherAttribute] = “Y”)
, [Country], [Product]
)
)
, ‘SomeData'[Country] = “FRA”)

‘SomeData’ is first filtered by [Country], which only returns the rows for France. These data should then be filtered according to the value in [OtherAttribute], then be summarized by [Country] and [Product]. The one tuple remaining (“FRA”, “B”) should then be used to  filter ‘SomeData’ – overriding our initial filter. This is not the case. Here, the product will only get filtered by [Country].

Once again, replacing ‘SomeData’ in the first argument of the innermost CALCULATETABLE expression with VALUES(‘SomeData’) will produce the expected results.

In that case, however, the result of the query does not seem to depend on the order of the columns in the Excel table.

Measures display the same behavior

Example with COUNTROWS

Do measures display the same behavior? Unfortunately, yes.

Let us consider this measure:

[Fine] :=

    CALCULATE( COUNTROWS( VALUES(‘SomeData’) ),
SUMMARIZE(
FILTER( ‘SomeData’, [OtherAttribute] = “Y”)
, [Country],[Product]
)
)

This measure works as expected:

Fine
A B Total
FRA 2 2
USA 2 2
Total 2 2 4

For the same reasons as in the previous sections, this measure does not:

[Not fine] :=

    CALCULATE( COUNTROWS( ‘SomeData’ ),
SUMMARIZE(
FILTER( ‘SomeData’, [OtherAttribute] = “Y”)
, [Country],[Product]
)
)

The result:

Not fine
A B Total
FRA 2 4
USA 2 3
Total 3 3 6

Once again the grand total may change depending on the order of the columns in the linked table.

Example with SUM

You might think: “Not an issue. I always use VALUES when referencing tables or columns”.

Do  you?

Standard aggregate functions like COUNT, COUNTA, SUM, … only accept a direct reference to a column.

To check this, let us add a new column to our table:

Country Product OtherAttribute CheckColumn
USA A Y 100000000
FRA B Y 20000000
USA C N 3000000
FRA C N 400000
UK A N 50000
GER B N 6000
USA A N 700
FRA B N 80
FRA D N 9

Now, let us observer the same unexpected behavior.

[Sum fails] : =

CALCULATE(   SUM(‘SomeData'[CheckColumn]   ),
SUMMARIZE(
FILTER( ‘SomeData’, [OtherAttribute] = “Y”)
, [Country],[Product]
)
)

The results:

Sum fails
A B Total
FRA 20000080 20400089
USA 100000700 103000700
Total 100050700 20006080 120056780

Note that it should be easy to track which row is in the context for each cell:

For example, the total for FRA is 20400089.  This clearly indicates the last two rows in our previous table (where [CheckColumn] = 80 and 9 ) were included in our table.

Here is one way to make it work as expected.

[Sum works]: =

CALCULATE(
CALCULATE( SUM(‘SomeData'[CheckColumn]   ), VALUES(‘SomeData’)) ,
SUMMARIZE(
FILTER( ‘SomeData’, [OtherAttribute] = “Y”)
, [Country],[Product]
)
)

Sum works
A B Total
FRA 20000080 20000080
USA 100000700 100000700
Total 100000700 20000080 120000780

Notes on “Usage of dates argument in a row context” by Marco Russo

Marco Russo recently blogged about the usage of dates arguments in a row context.  The measure he presents as producing “wrong results”, actually produces the results I expected. For this reason, I had a hard time understanding the motivation behind his article and decided to write a follow-up in case anyone else missed his very subtle explanations about context in DAX expression. If you have not done it yet, I recommend you read his entry, before and/or after reading mine.

So, suppose you have a table called OrderDate with a Date column and one row for each day, and define the following measures:

LastDay : = LASTDATE(OrderDate[Date])

NumLastDay := COUNTROWS(
FILTER(OrderDate,
OrderDate[Date] = [LastDay]
)
)

If you evaluate the measure NumLastDay for the month of January, for example, you will get 31 as a result. As already explained by Marco, FILTER iterates the OrderDate table: It creates a row context for every row of the OrderDate table present in the current filter context. The measure LastDay will be evaluated for every single row(day). Hence, the expression OrderDate[Date] = [LastDay] will always return TRUE.

Contrary to Marco, I was expecting this. Also, you can get the same result if you define LastDay as
LastDay := MAX(OrderDate[Date])
That is why I did not immediately get his point, and what was so special about the LASTDATE function.

However, LASTDATE and MAX behave differently when used directly within the FILTER formula:

DirectCall – MAX := COUNTROWS(
FILTER(OrderDate,
OrderDate[Date] = MAX(OrderDate[Date])
)
)

will return 1 when evaluated for the month of January. The evaluation of OrderDate[Date] within the MAX expression is done for the current filter context, disregarding the row context created by the FILTER expression.

On the contrary,

DirectCall – LASTDATE := COUNTROWS(
FILTER(OrderDate,
OrderDate[Date] = LASTDATE(OrderDate[Date])
)
)

will return 31 when evaluated for the month of January.

The filter expression
OrderDate[Date] = LASTDATE(OrderDate[Date])
actually behaves like the following expressions
OrderDate[Date] = CALCULATE(MAX(OrderDate[Date]))
OrderDate[Date] = CALCULATE(MAXX(OrderDate,[Date]))
OrderDate[Date] = MAXX(CALCULATETABLE(OrderDate),[Date]))
OrderDate[Date] = CALCULATE(LASTDATE(OrderDate[Date]))
OrderDate[Date] = LASTDATE(CALCULATETABLE(OrderDate[Date]))

where the CALCULATE and CALCULATETABLE parts create a new filter context from the current row context, for the evaluation of OrderDate[Date].

That is the point Marco Russo makes in his article about the LASTDATE function. He presents two solutions to calculate a measure for the last day within the current filter context (see his article).

Here is mine, by the way

=CALCULATE(
COUNTROWS(OrderDate),
LASTDATE(OrderDate[Date])
)

DAX Queries in Excel 2013 : using ROLLUP to cover some UNION scenarios

DAX does not offer a UNION function or operator. There are a few scenarios where this would be useful.

The scenario

Consider the following table called Games.

image

This is a very simple table. Basically, this table represents a relation between players, which can be stated as “Player 1 played against Player 2”. For now, I will ignore the meaning of why one player is listed in one column or the other.

In order to analyze this data, we will need a Player dimension, representing the set of all players, regardless of whether they were labeled as Player 1 or Player 2.

If the Games table came from a SQL connection, then we might as well use the following query to get the desired result.

SELECT [Player 1] AS Player FROM Games
UNION
SELECT [Player 2] FROM Games

The table would look like this.

image

(Remember that UNION only returns distinct rows).

A DAX solution

I already mentioned Excel 2013 supports DAX queries,  and the resulting tables can be re-used in the data model. Let us take a look at what we need to achieve the desired result.

First thing we need to care about, is that our result set might have more entries than our Games table. We will need to generate at least one row per distinct value in player 1 and 1 row per distinct value in player 2.

Now, imagine you create a PivotTable based on the Games table and put [Player 1] and [Player 2] on rows. This is what you will get:

image

For every distinct value of [Player 1], Excel will create a group, list all the values for [Player 2] within this group, and will add a row for the sub-total.

Since we cannot use the body range of a pivot table, as a data source, we will do this with a DAX query.

SUMMARIZE and ROLLUP will allow us to mimic this behavior.

The following DAX statement

EVALUATE
SUMMARIZE(
Games
, [Player 1]
, ROLLUP(  [Player 2])
, “Player”
, IF (ISSUBTOTAL(Games[Player 2]),[Player 1] , [Player 2])
)

will return the following table:

image

Note that ROLLUP([Player 2]) instructs PowerPivot to generate a subtotal row for the current value of [Player 1]. We also create a calculated column named Player, that will just take the value of [Player 2] for “regular” rows, and the value of [Player 1] for sub-totals. Also, ISSUBTOTAL allows us to differentiate subtotal rows from rows where [Player 2] is blank.

Finally, we just want to get the unique values in the Player column. For that, we will use, once again, the SUMMARIZE function.

The final query

EVALUATE
SUMMARIZE(
SUMMARIZE(
Games
, [Player 1]
, ROLLUP(  [Player 2])
, “Player”
, IF (ISSUBTOTAL(Games[Player 2]),[Player 1] , [Player 2])
)
,  [Player]
)

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