Unpack all arrays in a JSON column SQL Server 2019 - sql

Question

Say I have a table Schema.table with these columns
id | json_col
on the forms e.g
id=1
json_col ={"names":["John","Peter"],"ages":["31","40"]}
The lengths of names and ages are always equal but might vary from id to id (size is at least 1 but no upper limit).
How do we get an "exploded" table - a table with a row for each "names", "ages" e.g
id | names | ages
---+-------+------
1 | John | 31
1 | Peter | 41
2 | Jim | 17
3 | Foo | 2
.
.
I have tried OPENJSON and CROSS APPLY but the following gives any combination of names and ages which is not correct, thus I need to to a lot of filtering afterwards
SELECT *
FROM Schema.table
CROSS APPLY OPENJSON(Schema.table,'$.names')
CROSS APPLY OPENJSON(Schema.table,'$.ages')

Answer 1

Here's my suggestion
DECLARE #tbl TABLE(id INT,json_col NVARCHAR(MAX));
INSERT INTO #tbl VALUES(1,N'{"names":["John","Peter"],"ages":["31","40"]}')
,(2,N'{"names":["Jim"],"ages":["17"]}');
SELECT t.id
,B.[key] As ValueIndex
,B.[value] AS PersonNam
,JSON_VALUE(A.ages,CONCAT('$[',B.[key],']')) AS PersonAge
FROM #tbl t
CROSS APPLY OPENJSON(t.json_col)
WITH(names NVARCHAR(MAX) AS JSON
,ages NVARCHAR(MAX) AS JSON) A
CROSS APPLY OPENJSON(A.names) B;
The idea in short:
We use OPENJSON with a WITH clause to read names and ages into new json variables.
We use one more OPENJSON to "explode" the names-array
As the key is the value's position within the array, we can use JSON_VALUE() to read the corresponding age-value by its position.
One general remark: If this JSON is under your control, you should change this to an entity-centered approach (array of objects). Such a position dependant storage can be quite erronous... Try something like
{"persons":[{"name":"John","age":"31"},{"name":"Peter","age":"40"}]}

Answer 2

Conditional Aggregation along with applying CROSS APPLY might be used :
SELECT id,
MAX(CASE WHEN RowKey = 'names' THEN value END) AS names,
MAX(CASE WHEN RowKey = 'ages' THEN value END) AS ages
FROM
(
SELECT id, Q0.[value] AS RowArray, Q0.[key] AS RowKey
FROM tab
CROSS APPLY OPENJSON(JsonCol) AS Q0
) r
CROSS APPLY OPENJSON(r.RowArray) v
GROUP BY id, v.[key]
ORDER BY id, v.[key]
id | names | ages
---+-------+------
1 | John | 31
1 | Peter | 41
2 | Jim | 17
3 | Foo | 2
Demo
The first argument for OPENJSON would be a JSON column value, but not a table itself