I want to combine two datasets that share row keys but have different column keys. Visually, this looks like a JoinAcross on the row keys to me, but the column of row keys doesn't appear to be named. Can I perform this combination as a JoinAcross, or does it require a different function?

Also, the row keys are sometimes cumbersome--is there a way to demote them to normal rows?

Example of desired dataset combination

I have two datasets:

dsA = {<|"keyA" -> <|"key1" -> "a", "key2" -> "b"|>,
    "keyB" -> <|"key1" -> "c", "key2" -> "d"|>|>} // Dataset;
dsB= {<|"keyA" -> <|"key3" -> "a", "key4" -> "b"|>,
    "keyB" -> <|"key3" -> "d", "key4" -> "f"|>|>} // Dataset;

I want the join these such that I get a third dataset:

dsNew ={<|"keyA" -> <|"key1" -> "a", "key2" -> "b", "key3" -> "a", 
     "key4" -> "b"|>,
   "keyB" -> <|"key1" -> "c", "key2" -> "d", "key3" -> "d", 
     "key4" -> "f"|>|>} // Dataset

I have tried JoinAcross[ dsA, dsB, Keys[dsA]], but this produces unclear errors.

Example of desired Key demotion

I have dsA again. I want to "demote" its row keys, such that I get:

{<|"RowKeys" -> "keyA", "key1" -> "a", "key2" -> "b"|>,
  <|"RowKeys" -> "keyB", "key1" -> "c", "key2" -> "d"|>} // Dataset

2 Answers 2


Join at level 3:

dsC = Join[dsA, dsB, 3]

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For the second part of the question:

Apply[KeyValueMap[<|"RowKeys" -> #, #2|>&]] @ dsA 

enter image description here

Apply[KeyValueMap[<|"RowKeys" -> #, #2|>&]] @ dsC

enter image description here

  • $\begingroup$ The Apply method works for a two row dataset, but appears to fail for datasets with more than two rows. For example, if I say dsC=Join[dsA,dsB,3] and then use the above Apply method, I get a Failure: KeyValueMap[Association[RowKeys->#1,#2]&] called with 2 arguments; 1 argument is expected. $\endgroup$
    – KBL
    Aug 3, 2019 at 1:51
  • $\begingroup$ @KBL, posted what i get when I use Apply[KeyValueMap[<|"RowKeys" -> #, #2|>&]] on dsC on v12 /Wolfram Cloud. $\endgroup$
    – kglr
    Aug 3, 2019 at 2:01
  • $\begingroup$ you are right, I typed the wrong thing. What I meant to type was that the Apply method doesn't work when there are more than 2 rows--if I have dsD=Join[dsA,dsB] (without the 3) and type Apply[KeyValueMap[<|"RowKeys" -> #, #2|>&]]@dsD, I get KeyValueMap[Association[RowKeys->#1,#2]&] called with 2 arguments; 1 argument is expected. $\endgroup$
    – KBL
    Aug 5, 2019 at 16:01
  • $\begingroup$ @KBL, looks like changing Apply to Map works for both cases. $\endgroup$
    – kglr
    Aug 6, 2019 at 16:14
  • 1
    $\begingroup$ Thanks very much for the answers here and on the other question. As I posted on the other page, I found a nasty dataset wrinkle that was causing my dataset to be especially uncooperative. With your help, though, I have the answer. $\endgroup$
    – KBL
    Aug 6, 2019 at 18:39
    Merge[{Normal@dsA, Normal@dsB}, Join @@ # &] // Dataset

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