向 mysql json_arrayagg 函数添加条件

Question

I have a json query that gives me json of a joined table of person and pets:

SELECT json_object(
  'personId', p.id,
  'pets', json_arrayagg(json_object(
    'petId', pt.id,
    'petName', pt.name
  ))
  )
FROM person p LEFT JOIN pets pt
ON p.id = pt.person_id
GROUP BY p.id;

my issue is that person can have 0 or more pets, and when a person have 0 pets I get list with 1 empty pet, and what I would like to get in that case is empty list.

this is what I get:

{
  "personId": 1,
  "pets": [
    {
      "petId": null,
      "petName": ""
    }
  ]
}

and I need:

{
  "personId": 1,
  "pets": []
}

is that possible?

Answer 1

The problem is that LEFT JOIN still returns columns from the table you're joining with, it just sets their values to NULL .

You can use IF to test COUNT(pt.id) , as this won't count null values.

SELECT json_object(
  'personId', p.id,
  'pets', IF(COUNT(pt.id) = 0, JSON_ARRAY(),
             json_arrayagg(json_object(
                'petId', pt.id,
                'petName', pt.name
                )
            ))
  )
FROM person p LEFT JOIN pets pt
ON p.id = pt.person_id
GROUP BY p.id;

Answer 2

Another possibility is to put the aggregation in a correlated subquery and use coalesce() to replace it with an empty array if no rows exist.

SELECT json_object('personID', p.id,
                   'pets', coalesce((SELECT json_arrayagg(json_object('petId', t.id,
                                                                      'petName', t.name))
                                            FROM pets t
                                            WHERE t.person_id = p.id),
                                    json_array()))
       FROM person p;

Answer 3

Adding another option:

select IFNULL( /*expression of select ...JSON_ARRAYAGG(
                                    JSON_OBJECT(....*/,JSON_ARRAY()) jarrayaggAlias

Cleaner below:

IFNULL( expression, ,JSON_ARRAY()) jarrayaggAlias

Result:

/* { jarrayaggAlias: []  }*/

If you swap alt_value for IFNULL to select [] , then all your results will be stringyfied.