Dataset Association of Association vs Hierarchical data

Question

I have the following dataset:

Dataset[
 <|1 -> <|"High School" -> 96, "Graduate" -> 138, "Uneducated" -> 58, 
"College" -> 53, "Unknown" -> 75, "Post-Graduate" -> 41, 
"Doctorate" -> 1|>, 
2 -> <|"Uneducated" -> 185, "Graduate" -> 382, "College" -> 130, 
 "High School" -> 265, "Unknown" -> 163, "Post-Graduate" -> 59, 
 "Doctorate" -> 57|>, 
3 -> <|"High School" -> 481, "Uneducated" -> 366, "Graduate" -> 784, 
"Unknown" -> 374, "Post-Graduate" -> 118, "College" -> 251, 
"Doctorate" -> 98|>, 
 4 -> <|"High School" -> 540, "Graduate" -> 866, 
"Post-Graduate" -> 161, "Doctorate" -> 152, "Unknown" -> 454, 
"College" -> 268, "Uneducated" -> 433|>, 
5 -> <|"Graduate" -> 628, "Unknown" -> 293, "College" -> 224, 
"Uneducated" -> 278, "Doctorate" -> 93, "High School" -> 402, 
"Post-Graduate" -> 91|>, 
6 -> <|"Graduate" -> 256, "High School" -> 181, "Doctorate" -> 39, 
"College" -> 67, "Unknown" -> 123, "Uneducated" -> 140, 
"Post-Graduate" -> 44|>, 
7 -> <|"Unknown" -> 37, "Doctorate" -> 11, "High School" -> 46, 
"Graduate" -> 74, "College" -> 20, "Uneducated" -> 27, 
"Post-Graduate" -> 2|>, 8 -> <|"High School" -> 2|>|>
]

According to my understanding of the Dataset documentation, this should be displayed as a table where the numerical categories are the rows and the educational categories the columns. Instead it's displayed as a hierarchical data (rows of rows). Why is that?

Answer 1

To obtain a tabular rendering for a dataset, all rows must have the same number of columns, with the same set of keys, in the same order. But in our case the last association has fewer elements than the rest and the keys are in different orders in each row. Assuming that $ds contains the dataset:

$ds[Values /* (PadRight[#, Automatic, ""] &), Keys]

To get a tabular rendering, we must normalize the key order and fill in the blanks in that last row. KeyUnion will do this:

$ds[Keys[#] -> KeyUnion[Values[#]] & /* AssociationThread]

This technique will also work when multiple rows are missing values:

$ds2 = $ds[All, RandomSample[#, RandomInteger[Length[#]]] &];
$ds2[Keys[#] -> KeyUnion[Values[#]] & /* AssociationThread]