MySQL 8 Json document merge data from multiple rows - mysql

Question

I've JSON documents in multiple rows as
Row #1
{
"data": {
"level": 1,
"name": "xyz",
"property": "value",
"children": [
{
"level": 2,
"name": "xyz-1"
}
]
}
}
Row #2
{
"data": {
"children": [
{
"level": 2,
"name": "xyz-2"
},
{
"level": 2,
"name": "xyz-3"
}
]
}
}
Row #3
{
"data": {
"children": [
{
"level": 2,
"name": "xyz-4"
}
]
}
}
I want to use MySQL 8 JSON_MERGE_PRESERVE in such a way so I get the result
{
"data": {
"level": 1,
"name": "xyz",
"property": "value",
"children": [
{
"level": 2,
"name": "xyz-1"
},
{
"level": 2,
"name": "xyz-2"
},
{
"level": 2,
"name": "xyz-3"
},
{
"level": 2,
"name": "xyz-4"
}
]
}
}
I've tried
SELECT JSON_MERGE_PRESERVE(
'{ "data": { "level": 1, "name": "xyz", "property": "value", "children": [ { "level": 2, "name": "xyz-1" } ] } }',
'{ "data": { "children": [ { "level": 2, "name": "xyz-2" }, { "level": 2, "name": "xyz-3" } ] } }',
'{ "data": { "children": [ { "level": 2, "name": "xyz-4" } ] } }'
) as json;
But I want to SELECT the JSON data from the table and merge it, something like
[Not working]
SELECT JSON_MERGE_PRESERVE(a.data_json) from
(SELECT data_json FROM data_table
WHERE name = 'abc') as a
The error message is
Error Code: 1582. Incorrect parameter count in the call to native function 'JSON_MERGE_PRESERVE'

Answer 1

One of the many options you can evaluate and use is CTE (Common Table Expressions) - 13.2.13 WITH (Common Table Expressions):
WITH RECURSIVE `cte` AS (
SELECT
1 AS `row`,
`data_json`
FROM
`data_table`
WHERE
`id` = 1
UNION ALL
SELECT
`cte`.`row` + 1 AS `row`,
JSON_MERGE_PRESERVE(`cte`.`data_json`, `data_table`.`data_json`) AS `data_json`
FROM
`data_table`, `cte`
WHERE
`data_table`.`id` = `cte`.`row` + 1
)
SELECT
JSON_PRETTY(`data_json`)
FROM
`cte`
ORDER BY
`row` DESC
LIMIT 1;
See dbfiddle.