how to extract and zip values from json with SQL, mysql - mysql

Question

I struggle to extract the required values from the JSON column in MySQL.
Imagine the variable:
SET #j = '[{"a": 1, "b": 2},{"a": 3, "b": 4, "c": {"d": 4}}, {"a": 5}]';
and I need to extract and return a new JSON array containing only selected key-value pairs, eg.: only a's and b's and if the required key does not exist I want to return null for that key:
[{"a": 1, "b": 2},{"a": 3, "b": 4},{"a": 5, "b": null}]
thanks

Answer 1

Here's a solution using MySQL 8.0's JSON_TABLE() function:
mysql> SET #j = '[{"a": 1, "b": 2},{"a": 3, "b": 4, "c": {"d": 4}}, {"a": 5}]';
Query OK, 0 rows affected (0.00 sec)
mysql> select j.* from json_table(#j, '$[*]'
columns (a int path '$.a', b int path '$.b')) as j;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
| 3 | 4 |
| 5 | NULL |
+------+------+
3 rows in set (0.00 sec)
Based on this result, we can build a new JSON array of objects:
mysql> select json_arrayagg(json_object('a', j.a, 'b', j.b)) as new
from json_table(#j, '$[*]'
columns (a int path '$.a', b int path '$.b')) as j;
+-----------------------------------------------------------+
| new |
+-----------------------------------------------------------+
| [{"a": 1, "b": 2}, {"a": 3, "b": 4}, {"a": 5, "b": null}] |
+-----------------------------------------------------------+
If you are using MySQL 5.x, I would recommend just fetch the whole JSON document to a client application where you can deserialize it into data structures to manipulate in code.