How to select a particular object from a postgres jsonb column - postgresql

Question

I am storing the assessments of students in a jsonb column call assessment, every row in the database stores the assessments of students for a particular subject, and i am trying to get a query that will help retrieve:
1. A student assessment for a particular subject.
2. All the student assessments for every subject offer by the student.
Here is a same of the result i get when i select all records from the table.
[
{
"id": "22670f29-1437-4af1-b907-f6940377a851",
"sessionId": 3,
"sectionId": 4,
"subjectId": 8,
"assessment": [
{
"exam": 50,
"grdae": "A",
"total": 79,
"position": "First",
"student_id": 2,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Aristotle"
},
{
"exam": 50,
"grdae": "B",
"total": 69,
"position": "Third",
"student_id": 3,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Elvis"
}
]
},
{
"id": "beca9d97-0d0a-4d89-b8c5-c9a6fed812a5",
"sessionId": 3,
"sectionId": 4,
"subjectId": 10,
"assessment": [
{
"exam": 50,
"grdae": "A",
"total": 79,
"position": "First",
"student_id": 2,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Aristotle"
},
{
"exam": 50,
"grdae": "B",
"total": 69,
"position": "Third",
"student_id": 3,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Elvis"
}
]
},
{
"id": "1ed9c3d6-2482-4b1d-ab59-b77a8933699c",
"sessionId": 3,
"sectionId": 4,
"subjectId": 9,
"assessment": [
{
"exam": 50,
"grdae": "A",
"total": 90,
"position": "First",
"student_id": 2,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Aristotle"
},
{
"exam": 50,
"grdae": "B",
"total": 69,
"position": "Third",
"student_id": 3,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Elvis"
}
]
}
]
This result contains assessment for the different subjects, with subjectIds, 8, 9 ,and 10 for two students with ids 2 and 3.
NOTE: sessionID,SectionID and subjectId are also columns in the table call assessment.
I should be able to get this object in return if i want a student with id 2 assessment for a subjectId of 9 and sessionId is 3 and also sectionId is 4:
{
"exam": 50,
"grdae": "A",
"total": 90,
"position": "First",
"student_id": 2,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Aristotle"
}
And i should be a to get this result, if i want a student with id 2 assessment for all three subjectIds, i.e id 8, 9, 10, and sessionId is 3 and also sectionId is 4:
[
{
"exam": 50,
"grdae": "A",
"total": 90,
"position": "First",
"student_id": 2,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Aristotle"
},
{
"exam": 50,
"grdae": "A",
"total": 79,
"position": "First",
"student_id": 2,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Aristotle"
},
{
"exam": 50,
"grdae": "A",
"total": 79,
"position": "First",
"student_id": 2,
"assessment_1": 9,
"assessment_2": 17,
"assessment_4": 5,
"student_name": "Anana Aristotle"
}
]

Answer 1

Minimized your data in the fiddle to:
[
{
"sessionId": 3,
"sectionId": 4,
"subjectId": 8,
"assessment": [
{
"student_id": 2
},
{
"student_id": 3
}
]
},
{
"sessionId": 3,
"sectionId": 4,
"subjectId": 10,
"assessment": [
{
"student_id": 2
},
{
"student_id": 3
}
]
},
{
"sessionId": 3,
"sectionId": 4,
"subjectId": 9,
"assessment": [
{
"student_id": 2
},
{
"student_id": 3
}
]
}
]
demo:db<>fiddle
The first step is to unnest the relevant id information:
SELECT
assessments,
(elems ->> 'sessionId')::int as session_id,
(elems ->> 'sectionId')::int as section_id,
(elems ->> 'subjectId')::int as subject_id,
(assessments ->> 'student_id')::int AS student_id
FROM
mytable,
json_array_elements(jsondata) as elems,
json_array_elements(elems -> 'assessment') as assessments
This results in:
assessments | session_id | section_id | subject_id | student_id
:--------------- | ---------: | ---------: | ---------: | ---------:
{"student_id":2} | 3 | 4 | 8 | 2
{"student_id":3} | 3 | 4 | 8 | 3
{"student_id":2} | 3 | 4 | 10 | 2
{"student_id":3} | 3 | 4 | 10 | 3
{"student_id":2} | 3 | 4 | 9 | 2
{"student_id":3} | 3 | 4 | 9 | 3
The json_array_elements() function unnest the json array into one record per array element. Because there are two nested arrays, you need to call this function twice. You can get the values of each level as you can see within the SELECT list.
After that you are able to filter whatever you want within the WHERE clause. The resulting records (the relevant assessment objects) can be grouped with json_agg() aggregate function:
SELECT
json_agg(assessments)
FROM (
-- query from above
) s
WHERE session_id IN (3)
AND section_id IN (4)
AND subject_id IN (9) -- only one subject
AND student_id IN (2)
or
SELECT
json_agg(assessments)
FROM (
-- query from above
) s
WHERE session_id IN (3)
AND section_id IN (4)
AND subject_id IN (8,9,10) -- more subjects
AND student_id IN (2)