Update How-to-get-result-set-suitable-for-graph-visualisation.md

How-to-get-result-set-suitable-for-graph-visualisation.md

@@ -1 +1,101 @@
-# How to build result set suitable for graph visualisation
+# How to get result set suitable for graph visualisation
+
+There are some requirements for result sets if you want to build 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:
+
+house:
+| name       | points |
+| ---------- | ------ |
+| Gryffindor | 100    |
+| Hufflepuff | 90     |
+| Ravenclaw  | 95     |
+| Slytherin  | 80     |
+
+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][1] function to form JSONs. The result set should contain both nodes and edges and we have to provide a field indicating that (0 - for nodes and 1 - for edges). Let's provide it as 'object_type':
+
+```sql
+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 first union will be used as node records and the second one - as edges.
+
+Then we need to provide an ID for each node. For students - `id` and for houses - `name`: 
+
+```sql
+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 record must specify where it starts and where it ends. Let's provide it as `source` and `target`.
+Provide values that are used as node IDs. In our case it's house names and student IDs:
+
+```sql
+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 won't be meaningfull without lables.
+Also, it would be nice to distinguish house nodes from student nodes e.g. by color.
+Let's put additional fields - `label` and `type` that can be used in graph styling.
+
+```sql
+SELECT json_object('object_type', 0, 'node_id', name, 'label', name, 'type', 'house')
+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:
+
+| json_object('object_type', 0, 'node_id', name, 'label', name, 'type', 'house') |
+| ------------------------------------------------------------------------------ |
+| {"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}                        |