Linux CentOS 6.5

But should JSON_KEYS return keys or unique keys, according to the standard? If it's just keys, and while duplicate keys are only discouraged but not prohibited, MariaDB behavior seems justified – there are indeed two keys "c1", so it returns two.

That is a valid point, but I'm not sure there is a good standard. MySQL works differently returning just 1 key. Now, my reasoning for voting for the MySQL behavior is that even though there are two keys, there is actually just 1 value, i.e. you can not access the second or following values with the same key. So if there is just 1 value there is in essence only one key/value pair. I tested how Jansson and PHP treats this, and PHP ignores the duplicates although stick with the last, instead of the first, value.

I have no opinion on the subject, so I'm assigning it to holyfoot to consult with serg and decide what's best.

Just please note that if you decide to change it, some other functions need to be changed accordingly, so they provide consistent results. For example,

MariaDB [test]> select json_length('{"a":1,"a":2,"b":3}');

+------------------------------------+

| json_length('{"a":1,"a":2,"b":3}') |

+------------------------------------+

|                                  3 |

+------------------------------------+

1 row in set (0.00 sec)

MariaDB [test]> select @@version;

+----------------------+

| @@version            |

+----------------------+

| 10.2.6-MariaDB-debug |

+----------------------+

1 row in set (0.00 sec)

MySQL [test]> select json_length('{"a":1,"a":2,"b":3}');

+------------------------------------+

| json_length('{"a":1,"a":2,"b":3}') |

+------------------------------------+

|                                  2 |

+------------------------------------+

1 row in set (0.01 sec)

MySQL [test]> select @@version;

+--------------+

| @@version    |

+--------------+

| 5.7.17-debug |

+--------------+

1 row in set (0.00 sec)

That's not a bug.
Mariadb made a good decision to allow duplicate keys, that's natural fact in daily life.
For example, students last name in a class, maybe have 1+ students named "John".
As for this good feature, lot's of user prefer to Mariadb instead of Mysql.

michiv: In your example, John is not a key but a value. Multiple values are fine. For example, if we have a class of students, this is valid and is not in contradiction to what I am looking for here:

{"classname": "JSON with MariaDB",

  "students": [{"firstname": "John", "Lastname": "Doe"},{"Firstname": "John", "Lastname": "Foo"}]

}

The following is the situation I am looking at:

{"classname": "JSON with MariaDB",

  "students": {"name": "John Doe", "name": "John Foo"}

}

In the last example, which on the two "name" attributes, when I ask for it, do we want? The first, the second or both.

http://lists.askmonty.org/pipermail/commits/2017-August/011360.html