How do I generate nested json objects using mysql native json functions? - mysql

Question

Using only the native JSON fuctions (no PHP, etc) in MySQL version 5.7.12 (section 13.16 in the manual) I am trying to write a query to generate a JSON document from relational tables that contains a sub object. Given the following example:
CREATE TABLE `parent_table` (
`id` int(11) NOT NULL,
`desc` varchar(20) NOT NULL,
PRIMARY KEY (`id`)
);
CREATE TABLE `child_table` (
`id` int(11) NOT NULL,
`parent_id` int(11) NOT NULL,
`desc` varchar(20) NOT NULL,
PRIMARY KEY (`id`,`parent_id`)
);
insert `parent_table` values (1,'parent row 1');
insert `child_table` values (1,1,'child row 1');
insert `child_table` values (2,1,'child row 2');
I am trying to generate a JSON document that looks like this:
[{
"id" : 1,
"desc" : "parent row 1",
"child_objects" : [{
"id" : 1,
"parent_id" : 1,
"desc" : "child row 1"
}, {
"id" : 2,
"parent_id" : 1,
"desc" : "child row 2"
}
]
}]
I am new to MySQL and suspect there is a SQL pattern for generating nested JSON objects from one to many relationships but I'm having trouble finding it.
In Microsoft SQL (which I'm more familiar with) the following works:
select
[p].[id]
,[p].[desc]
,(select * from [dbo].[child_table] where [parent_id] = [p].[id] for json auto) AS [child_objects]
from [dbo].[parent_table] [p]
for json path
I attempted to write the equivalent in MySQL as follows:
select json_object(
'id',p.id
,'desc',p.`desc`
,'child_objects',(select json_object('id',id,'parent_id',parent_id,'desc',`desc`)
from child_table where parent_id = p.id)
)
from parent_table p;
select json_object(
'id',p.id
,'desc',p.`desc`
,'child_objects',json_array((select json_object('id',id,'parent_id',parent_id,'desc',`desc`)
from child_table where parent_id = p.id))
)
from parent_table p
Both attempts fail with the following error:
Error Code: 1242. Subquery returns more than 1 row

Answer 1

The reason you are getting these errors is that the parent json object is not expecting a result set as one of its inputs, you need to have simple object pairs like {name, string} etc bug report - may be available in future functionality... this just means that you need to convert your multi row results into a concatination of results separated by commas and then converted into a json array.
You almost had it with your second example.
You can achieve what you are after with the GROUP_CONCAT function
select json_object(
'id',p.id
,'desc',p.`desc`
,'child_objects',json_array(
(select GROUP_CONCAT(
json_object('id',id,'parent_id',parent_id,'desc',`desc`)
)
from child_table
where parent_id = p.id))
)
from parent_table p;
This almost works, it ends up treating the subquery as a string which leaves the escape characters in there.
'{\"id\": 1,
\"desc\": \"parent row 1\",
\"child_objects\":
[\"
{\\\"id\\\": 1,
\\\"desc\\\": \\\"child row 1\\\",
\\\"parent_id\\\": 1
},
{\\\"id\\\": 2,
\\\"desc\\\": \\\"child row 2\\\",
\\\"parent_id\\\": 1}\"
]
}'
In order to get this working in an appropriate format, you need to change the way you create the JSON output as follows:
select json_object(
'id',p.id
,'desc',p.`desc`
,'child_objects',(select CAST(CONCAT('[',
GROUP_CONCAT(
JSON_OBJECT(
'id',id,'parent_id',parent_id,'desc',`desc`)),
']')
AS JSON) from child_table where parent_id = p.id)
) from parent_table p;
This will give you the exact result you require:
'{\"id\": 1,
\"desc\": \"parent row 1\",
\"child_objects\":
[{\"id\": 1,
\"desc\": \"child row 1\",
\"parent_id\": 1
},
{\"id\": 2,
\"desc\": \"child row 2\",
\"parent_id\": 1
}]
}'

Answer 2

For MariaDb, CAST AS JSON does not work. But JSON_EXTRACT may be used to convert a string to a JSON object:
select json_object(
'id',p.id
,'desc',p.`desc`
,'child_objects',JSON_EXTRACT(IFNULL((select
CONCAT('[',GROUP_CONCAT(
json_object('id',id,'parent_id',parent_id,'desc',`desc`)
),']')
from child_table where parent_id = p.id),'[]'),'$')
) from parent_table p;

Answer 3

I tried group_concat solution but I found it's problems larger string because of group_concat limitations (group_concat_max_len).
I wrote the new function resolve the problem about converting a string to JSON object as bellow and how to use it.
Tested on MariaDB 10.5.12
Usage: https://i.stack.imgur.com/cWfd7.jpg
CREATE FUNCTION `ut_tf_array`(input_json longtext) RETURNS longtext CHARSET utf8mb4 COLLATE utf8mb4_unicode_ci
COMMENT 'Function for transform json array agg'
BEGIN
DECLARE transformed_data_list longtext ;
DECLARE record longtext ;
DECLARE i_count int ;
DECLARE i_count_items int ;
SET i_count = 0;
SET i_count_items = JSON_LENGTH(JSON_EXTRACT(input_json,'$'));
SET transformed_data_list = '[]';
-- return array with length = zero
IF input_json is NULL THEN
RETURN transformed_data_list;
END IF;
WHILE i_count < i_count_items DO
-- fetch into record
SELECT JSON_EXTRACT( JSON_EXTRACT( input_json ,'$') , CONCAT('$[',i_count,']')) INTO record;
-- append to transformed_data_list
SELECT JSON_ARRAY_APPEND(transformed_data_list, '$', JSON_EXTRACT(record, '$')) into transformed_data_list;
SET i_count := i_count + 1;
END WHILE;
-- done
RETURN transformed_data_list;
END

Answer 4

Below Query works for me.
SELECT JSON_ARRAYAGG(JSON_OBJECT('Id', p.id, 'desc', p.`desc`, 'child_objects', temp_json)) AS json_value
FROM (
SELECT p.id, p.`desc`,
JSON_ARRAYAGG(JSON_OBJECT('id', p.id, 'parent_id', p.parent_id, 'desc', p.`desc`)) AS temp_json
FROM parent_table p
GROUP BY p.id
) t;