Syntax
------

JSON_LENGTH(json_doc[, path])

Description
-----------

Returns the length of a JSON document, or, if the optional path argument is
given, the length of the value within the document specified by the path.

Returns NULL if any of the arguments argument are null or the path argument
does not identify a value in the document.

An error will occur if the JSON document is invalid, the path is invalid or if
the path contains a * or ** wildcard.

Length will be determined as follow:

* A scalar's length is always 1.
* If an array, the number of elements in the array.
* If an object, the number of members in the object.

The length of nested arrays or objects are not counted.

Examples
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URL: https://mariadb.com/kb/en/json_length/