JSON functions were added in MariaDB 10.2.3.
 
Syntax
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JSON_LENGTH(json_doc[, path])
 
Description
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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/