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121 lines
4.5 KiB
Markdown
121 lines
4.5 KiB
Markdown
# How to get a result set suitable for graph visualisation
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There are some [requirements for result sets][1] if you want to make a graph.
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Here is an example of building a query that returns a result set appropriate for graph
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visualisation.
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Let's say, you have 2 tables:
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1. `house`:
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| name | points |
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| ---------- | ------ |
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| Gryffindor | 100 |
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| Hufflepuff | 90 |
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| Ravenclaw | 95 |
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| Slytherin | 80 |
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2. `student`:
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| id | name | house |
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| -- | -------------- | ---------- |
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| 1 | Harry Potter | Gryffindor |
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| 2 | Ron Weasley | Gryffindor |
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| 3 | Draco Malfoy' | Slytherin |
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| 4 | Luna Lovegood | Ravenclaw |
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| 5 | Cedric Diggory | Hufflepuff |
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Each student belongs to a certain house.
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Let's say you want to build a graph with houses and students as nodes, where each house is linked
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with its students.
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We are going to use [json_object][2] function to form JSON documents. The result set should contain
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both nodes and edges and we have to provide a field indicating if the document represents a node
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(0) or and edge (1). Let's provide it as `object_type`:
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```sql
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SELECT json_object('object_type', 0)
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FROM house
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UNION ALL
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SELECT json_object('object_type', 0)
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FROM student
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UNION ALL
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SELECT json_object('object_type', 1)
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FROM student
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```
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Note that we included `student` table twice. That is because the table contains not only students
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but also their relationship to houses. So the records from the first union of `student` are used
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as nodes and from the second one - as edges.
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Then we need to provide an ID for each node. Let's put it in `node_id` field. The `node_id` value
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for students is taken from `id` column and for houses - from `name`:
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```sql
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SELECT json_object('object_type', 0, 'node_id', name)
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FROM house
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UNION ALL
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SELECT json_object('object_type', 0, 'node_id', id)
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FROM student
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UNION ALL
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SELECT json_object('object_type', 1)
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FROM student
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```
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Each edge document must provide a node id where the edge begins and where it ends. Let's put it in
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`source` and `target`:
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```sql
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SELECT json_object('object_type', 0, 'node_id', name)
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FROM house
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UNION ALL
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SELECT json_object('object_type', 0, 'node_id', id)
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FROM student
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UNION ALL
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SELECT json_object('object_type', 1, 'source', house, 'target', id)
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FROM student
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```
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Basically, that is enough to build a graph. But it is not very meaningful without labels.
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Also, it would be nice to distinguish house nodes from student nodes by color.
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Let's put additional fields `label` and `type` that can be used in graph styling.
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```sql
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SELECT json_object('object_type', 0, 'node_id', name, 'label', name, 'type', 'house') AS graph_object
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FROM house
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UNION ALL
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SELECT json_object('object_type', 0, 'node_id', id, 'label', name, 'type', 'student')
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FROM student
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UNION ALL
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SELECT json_object('object_type', 1, 'source', house, 'target', id)
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FROM student
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```
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Run the query, the result set will look like this:
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| graph_object |
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| ------------------------------------------------------------------------------ |
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| {"object_type":0,"node_id":"Gryffindor","label":"Gryffindor","type":"house"} |
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| {"object_type":0,"node_id":"Hufflepuff","label":"Hufflepuff","type":"house"} |
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| {"object_type":0,"node_id":"Ravenclaw","label":"Ravenclaw","type":"house"} |
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| {"object_type":0,"node_id":"Slytherin","label":"Slytherin","type":"house"} |
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| {"object_type":0,"node_id":1,"label":"Harry Potter","type":"student"} |
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| {"object_type":0,"node_id":2,"label":"Ron Weasley","type":"student"} |
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| {"object_type":0,"node_id":3,"label":"Draco Malfoy","type":"student"} |
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| {"object_type":0,"node_id":4,"label":"Luna Lovegood","type":"student"} |
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| {"object_type":0,"node_id":5,"label":"Cedric Diggory","type":"student"} |
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| {"object_type":1,"node_source":"Gryffindor","target":1} |
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| {"object_type":1,"node_source":"Gryffindor","target":2} |
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| {"object_type":1,"node_source":"Slytherin","target":3} |
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| {"object_type":1,"node_source":"Ravenclaw","target":4} |
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| {"object_type":1,"node_source":"Hufflepuff","target":5} |
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Now in the graph editor, we can set mapping of the result set documents into node and edge
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properties, set graph styles and get the following visualisation:
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[1]: ./graph#requirements-for-result-set
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[2]: https://sqlite.org/json1.html#jobj
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