How to get a result set suitable for graph visualisation

There are some requirements for result sets if you want to make a graph. Here is an example of building a query that returns a result set appropriate for graph visualisation.

Let's say, you have 2 tables:

1. house:

   name	points
   Gryffindor	100
   Hufflepuff	90
   Ravenclaw	95
   Slytherin	80

2. student:

   id	name	house
   1	Harry Potter	Gryffindor
   2	Ron Weasley	Gryffindor
   3	Draco Malfoy'	Slytherin
   4	Luna Lovegood	Ravenclaw
   5	Cedric Diggory	Hufflepuff

Each student belongs to a certain house. Let's say you want to build a graph with houses and students as nodes, where each house is linked with its students.

We are going to use json_object function to form JSON documents. The result set should contain both nodes and edges and we have to provide a field indicating if the document represents a node (0) or and edge (1). Let's provide it as object_type:

SELECT json_object('object_type', 0)
FROM house
UNION ALL
SELECT json_object('object_type', 0)
FROM student
UNION ALL
SELECT json_object('object_type', 1)
FROM student

Note that we included student table twice. That is because the table contains not only students but also their relationship to houses. So the records from the first union of student are used as nodes and from the second one - as edges.

Then we need to provide an ID for each node. Let's put it in node_id field. The node_id value for students is taken from id column and for houses - from name:

SELECT json_object('object_type', 0, 'node_id', name)
FROM house
UNION ALL
SELECT json_object('object_type', 0, 'node_id', id)
FROM student
UNION ALL
SELECT json_object('object_type', 1)
FROM student

Each edge document must provide a node id where the edge begins and where it ends. Let's put it in source and target:

SELECT json_object('object_type', 0, 'node_id', name)
FROM house
UNION ALL
SELECT json_object('object_type', 0, 'node_id', id)
FROM student
UNION ALL
SELECT json_object('object_type', 1, 'source', house, 'target', id)
FROM student

Basically, that is enough to build a graph. But it is not very meaningful without labels. Also, it would be nice to distinguish house nodes from student nodes by color. Let's put additional fields label and type that can be used in graph styling.

SELECT json_object('object_type', 0, 'node_id', name, 'label', name, 'type', 'house') AS graph_object
FROM house
UNION ALL
SELECT json_object('object_type', 0, 'node_id', id, 'label', name, 'type', 'student')
FROM student
UNION ALL
SELECT json_object('object_type', 1, 'source', house, 'target', id)
FROM student

Run the query, the result set will look like this:

graph_object
{"object_type":0,"node_id":"Gryffindor","label":"Gryffindor","type":"house"}
{"object_type":0,"node_id":"Hufflepuff","label":"Hufflepuff","type":"house"}
{"object_type":0,"node_id":"Ravenclaw","label":"Ravenclaw","type":"house"}
{"object_type":0,"node_id":"Slytherin","label":"Slytherin","type":"house"}
{"object_type":0,"node_id":1,"label":"Harry Potter","type":"student"}
{"object_type":0,"node_id":2,"label":"Ron Weasley","type":"student"}
{"object_type":0,"node_id":3,"label":"Draco Malfoy","type":"student"}
{"object_type":0,"node_id":4,"label":"Luna Lovegood","type":"student"}
{"object_type":0,"node_id":5,"label":"Cedric Diggory","type":"student"}
{"object_type":1,"node_source":"Gryffindor","target":1}
{"object_type":1,"node_source":"Gryffindor","target":2}
{"object_type":1,"node_source":"Slytherin","target":3}
{"object_type":1,"node_source":"Ravenclaw","target":4}
{"object_type":1,"node_source":"Hufflepuff","target":5}

Now in the graph editor, we can set mapping of the result set documents into node and edge properties, set graph styles and get the following visualisation: