Similar questions asked here before:

Count items for a single key: jq count the number of items in json by a specific key

Calculate the sum of object values:How do I sum the values in an array of maps in jq?

Question

How to emulate the COUNT aggregate function which should behave similarly to its SQL original? Let's extend this question even more to include other regular SQL functions:

• COUNT
• SUM / MAX/ MIN / AVG
• ARRAY_AGG

The last one is not a standard SQL function - it's from PostgreSQL but is quite useful.

At input comes a stream of valid JSON objects. For demonstration let's pick a simple story of owners and their pets.

Model and data

Base relation: Owner

id name  age 1 Adams  25 2 Baker  55 3 Clark  40 4 Davis  31

Base relation: Pet

id name  litter owner_id10 Bella      4        120 Lucy       2        130 Daisy      3        240 Molly      4        350 Lola       2        460 Sadie      4        470 Luna       3        4

Source

From above we get a derivative relation Owner_Pet (a result of SQL JOIN of the above relations) presented in JSON format for our jq queries (the source data):

{ "owner_id": 1, "owner": "Adams", "age": 25, "pet_id": 10, "pet": "Bella", "litter": 4 }{ "owner_id": 1, "owner": "Adams", "age": 25, "pet_id": 20, "pet": "Lucy",  "litter": 2 }{ "owner_id": 2, "owner": "Baker", "age": 55, "pet_id": 30, "pet": "Daisy", "litter": 3 }{ "owner_id": 3, "owner": "Clark", "age": 40, "pet_id": 40, "pet": "Molly", "litter": 4 }{ "owner_id": 4, "owner": "Davis", "age": 31, "pet_id": 50, "pet": "Lola",  "litter": 2 }{ "owner_id": 4, "owner": "Davis", "age": 31, "pet_id": 60, "pet": "Sadie", "litter": 4 }{ "owner_id": 4, "owner": "Davis", "age": 31, "pet_id": 70, "pet": "Luna",  "litter": 3 }

Requests

Here are sample requests and their expected output:

• COUNT the number of pets per owner:

{ "owner_id": 1, "owner": "Adams", "age": 25, "pets_count": 2 }{ "owner_id": 2, "owner": "Baker", "age": 55, "pets_count": 1 }{ "owner_id": 3, "owner": "Clark", "age": 40, "pets_count": 1 }{ "owner_id": 4, "owner": "Davis", "age": 31, "pets_count": 3 }

• SUM up the number of whelps per owner and get their MAX (MIN/AVG):

{ "owner_id": 1, "owner": "Adams", "age": 25, "litter_total": 6, "litter_max": 4 }{ "owner_id": 2, "owner": "Baker", "age": 55, "litter_total": 3, "litter_max": 3 }{ "owner_id": 3, "owner": "Clark", "age": 40, "litter_total": 4, "litter_max": 4 }{ "owner_id": 4, "owner": "Davis", "age": 31, "litter_total": 9, "litter_max": 4 }

• ARRAY_AGG pets per owner:

{ "owner_id": 1, "owner": "Adams", "age": 25, "pets": [ "Bella", "Lucy" ] }{ "owner_id": 2, "owner": "Baker", "age": 55, "pets": [ "Daisy" ] }{ "owner_id": 3, "owner": "Clark", "age": 40, "pets": [ "Molly" ] }{ "owner_id": 4, "owner": "Davis", "age": 31, "pets": [ "Lola", "Sadie", "Luna" ] }