Equivalences between Tables, Maps, Graphs, and Sets

Sunday April 17, 2016

By tables, I mean the relational database or well-used spreadsheet system of data in rectangular grids of rows and columns.

| property_a  | property_b  |
| value_a_one | value_b_one |
| value_a_two | value_b_two |

By maps, I mean data in map data structures. Maps are also known as associative arrays or dictionaries. In JavaScript they're called objects, and JavaScript Object Notation (JSON) is a common format.

[{property_a: value_a_one, property_b: value_b_one},
 {property_a: value_a_two, property_b: value_b_two}]

By graphs, I mean nodes connected by edges. RDF, for example, has labels for both nodes and edges. One representation is triples of the form subject, predicate, object, where subject and object are nodes, and predicate is an edge.

id_one, property_a, value_a_one
id_one, property_b, value_b_one
id_two, property_a, value_a_two
id_two, property_b, value_b_two

A visual for this triple graph shows two components.

graph of triple data

Graphs can also have unlabeled edges, in which case one representation is doubles of the form subject, object.

id_one, id_a_one
id_a_one, property_a
id_a_one, value_a_one
id_one, id_b_one
id_b_one, property_b
id_b_one, value_b_one
id_two, id_a_two
id_a_two, property_a
id_a_two, value_a_two
id_two, id_b_two
id_b_two, property_b
id_b_two, value_b_two

A visual for one component of this double graph shows how the additional identity nodes expand the representation.

graph of double data

In this formulation, with non-identity nodes always leaf nodes, double graphs are equivalent to sets.

[{{property_a, value_a_one}, {property_b, value_b_one}},
 {{property_a, value_a_two}, {property_b, value_b_two}}]

There are a lot of close equivalences:

Tabular data can be converted to map data. With Python's pandas data frames this is as easy as dataframe.to_json().

Often maps can be complex in ways that don't correspond immediately to tabular data, but still it's often possible for example to convert JSON to CSV.

Graphs often work with doubles or triples directly as a storage format.

Automatically converting tabular or map data to/from triples is less common. I have a little toy data diff code that does a simple version of this. I think some parts of datomic's internals do related conversions.

Doubles are interesting but I don't know of any automatic converters to or from other formats.

Some considerations argue against the primacy of two-dimensional data tables, and some of these concerns are valid, but it is not insignificant that spreadsheet interfaces seem to work pretty well for a lot of humans, and probably even if relying on a different underlying structure tables should remain as an interface.

One complaint about spreadsheets in particular, though relational databases are not necessarily much better, is that people can and do choose layouts for their data that other people find appalling.

Proponents of "data cubes" might also suggest that two dimensions is not enough for some types of data.

Wickham's Tidy Data paper provides responses to both of these objections. In the first place, a wide range of table layouts are equivalent and can be converted to a standard "tidy" format if needed. There is no fundamental need to impose any given format in the name of aesthetics. In the second place, an arbitrary number of dimensions can be represented in a two-dimensional table, though such a representation may not be ideal for human users. There remains a question of how to provide a good interface.

It is awkward in a flat table to represent things that have multiple values. For example, with one row per song and one "artist" field, what do you do for a song that has two artists?

You might want to create a "second artist" field, which could be a solution to a different problem. What we really want is to have two entries each "in the same place" in the data.

Whatever the solution, there is a related interface issue: especially with a tabular display, how do we show the user that there are two values "in a cell"?

A technique common in relational databases hacks a graph approach into the tabular framework. It uses one table for songs, one table for artists, and a third table of "edges" connecting songs and artists together. This means you have to know about a lot of the tables' design. It can seem like overkill when the songs and artists are really just strings. And there's not an obvious solution to the interface problem.

You might want to use composite data types: instead of storing one simple value in the song-artist position, for example, store a list of values. This could be done with relational databases or spreadsheets, but it generally isn't done. It is easy to do with pandas data frames or in JSON.

Composite data values are probably worth having for some use cases. This comes with a new interface issue, distinct from the issue of an interface for multiple values. And it isn't a good solution to the original problem of allowing multiple values. Aside from lists in particular being ordered, there is a difference between something having multiple values, and something being one list.

Composite data like lists also introduce complexity that could be avoided. It is easy and natural with triple graphs (though not maps) to represent multiple values.

id_song_one, artist, a_jazz_band
id_song_one, artist, a_jazz_singer

The other side of multiple values is missing values. Tables, when realized as such, are "dense", meaning they have something at every intersection of row and column. Since we don't always have a value, a database might use NULL, or there could be special values like NA in R. With triple graphs (and maps) these aren't necessary as you always have a choice to not include a triple. Triples are naturally "sparse" for missing data (distinct from sparsity for zeros).

| property_a  | property_b  |
| value_a_one | NULL        |
| NULL        | value_b_two |
[{property_a: value_a_one},
 {property_b: value_b_two}]
id_one, property_a, value_a_one
id_two, property_b, value_b_two

Spreadsheets are not relational databases. Products like Excel implement a kind of discrete Cartesian quarter-plane within which simple data types can be placed. The spacial nature of the spreadsheet supports calculations like "add the value to the left of this one to the value three above" and having multiple tables "next to" one another.

A spreadsheet user may be thinking of tables, and some software functionality may support this, but for the most part structure in spreadsheets is hallucinated by the user. It is a testament to human abilities, combining remarkable gestalt perception with careful attention to detail, that anything useful is ever accomplished with spreadsheets.

It seems like giving spreadsheet software well-defined tables rather than vast expanses of empty cells would be a change worth making. Some tools, such as editdata, do this.

It's awkward for people to think about things without physical metaphor. Set literals have to be written in some order, for example, despite not having any order intrinsically. In the same way, tables have spacial baggage. Columns are always next to one another. Rows are before or after one another.

If you change the order of rows or columns, there is some change, but it isn't a very big change.

| property_a  | property_b  |
| value_a_one | value_b_one |
| value_a_two | value_b_two |
| property_b  | property_a  |
| value_b_one | value_a_one |
| value_b_two | value_a_two |

If you diff CSV files after re-ordering columns, every line of the file has changed. But in some sense the data hasn't changed at all.

I think the right way to conceptualize this is to see even a simple CSV file as a view of an underlying data set. While we usually don't make it explicit, the ordering of columns and rows is characteristic of the view and not the data.

With both tables and maps, sometimes the order of things has meaning that should really be in another field.

Separate though sometimes conflated with ordering is the idea of a unique identity for each row or map. By their distinctness they have an implicit identity separate from any values.

Especially when you're interested in comparing two versions of a data set, having an ID for each "row" is important. Otherwise you're in the fraught position of having to determine what changed and what was supposed to be the same, at the same time. This is impossible for at least some changes.

The dat project beta, for example, had an import -key option (see beta docs). The default behavior though was to automatically generate a unique ID, which is also what MongoDB does by default.

When specifying IDs by hand it is tempting to make choices that turn out to be bad, like using a name as an ID and finding later both that names can change and multiple things can have the same name.

Once there are unique IDs for rows/maps, comparison between versions easily discovers which have been added and removed, and those that remain can be compared to their partners.

Doubles and sets as a data representation are interesting but may not be very useful. There is nothing to distinguish properties from values. It could probably still be worked out by finding patterns of value occurrences, but this would be expensive along the same lines as matching up rows without IDs. It is a little poetic to think about expressing things like {smell, roses, raindrops} but in the end the sets and doubles don't directly convey all the same information as the other formats. A while ago I thought that doubles were sufficient, but I no longer think so.

My main interest in all this is dealing with the problem of versioning data, and the related problems of diffing and merging data. Often, only the most recent version of data is available or even stored anywhere, which I don't like.

For all the graphs here, I'm not considering duplicate edges.

I made the graph diagrams with draw.io.