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.

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.

(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:

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:

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]
)