MariaDB 10.6

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SET @json='{"a": {"z":true,"y":false}, "b": {"z":false,"y":false} }';
SELECT JSON_SEARCH(@json, 'all', 'true', NULL, '$.b') IS NOT  NULL;

JSON_SEARCH(@json, 'all', 'true', NULL, '$.b') IS NOT  NULL

0

SELECT JSON_SEARCH(@json, 'all', 'true', NULL, '$.a') IS NOT  NULL

JSON_SEARCH(@json, 'all', 'true', NULL, '$.a') IS NOT  NULL

1