In MariaDB, JSON_MERGE_PATCH() is a built-in function that merges two or more JSON documents and returns the result.

The JSON_MERGE_PATCH() function is an RFC 7396-compliant replacement for the JSON_MERGE() function, which has been deprecated.

Syntax

The syntax goes like this:

JSON_MERGE_PATCH(json_doc, json_doc[, json_doc] ...)

Where json_doc are the JSON documents to merge.

Example

Here’s an example to demonstrate.

SELECT JSON_MERGE_PATCH('{"name":"Wag"}', '{"type":"Dog"}');

Result:

+------------------------------------------------------+
| JSON_MERGE_PATCH('{"name":"Wag"}', '{"type":"Dog"}') |
+------------------------------------------------------+
| {"name": "Wag", "type": "Dog"}                       |
+------------------------------------------------------+

We can see that the two documents have been merged into one.

Here’s an example that merges three documents:

SELECT JSON_MERGE_PATCH(
    '{ "name" : "Wag" }', 
    '{ "type" : "Dog" }',
    '{ "score" : [ 9, 7, 8 ] }'
    ) AS Result;

Result:

+----------------------------------------------------+
| Result                                             |
+----------------------------------------------------+
| {"name": "Wag", "type": "Dog", "score": [9, 7, 8]} |
+----------------------------------------------------+

Arrays

The JSON_MERGE_PATCH() function does not merge arrays:

SELECT JSON_MERGE_PATCH(
    '[1,2,3]', 
    '[4,5,6]'
    ) AS Result;

Result:

+-----------+
| Result    |
+-----------+
| [4, 5, 6] |
+-----------+

This is in contrast to the (deprecated) JSON_MERGE() function and its synonym JSON_MERGE_PRESERVE(), both of which do merge arrays.

Format of the Result

You might have noticed that JSON_MERGE_PATCH() adds spaces to the resulting document. If this is a problem, you can use JSON_COMPACT() to remove the space.

Example:

SELECT 
    JSON_COMPACT(
        JSON_MERGE_PATCH(
            '{"name":"Wag"}', 
            '{"type":"Dog"}',
            '{"score":[9,7,8]}'
        )
    ) AS Result;

Result:

+---------------------------------------------+
| Result                                      |
+---------------------------------------------+
| {"name":"Wag","type":"Dog","score":[9,7,8]} |
+---------------------------------------------+

But if you need to go the other way and get extra formatting, such as indented structures, try the JSON_DETAILED() function.

SELECT 
    JSON_DETAILED(
        JSON_MERGE_PATCH(
            '{ "name" : "Wag" }', 
            '{ "type" : "Dog" }',
            '{ "score" : [ 9, 7, 8 ] }'
        )
    ) AS Result;

Result:

+---------------------------------------+
| Result                                |
+---------------------------------------+
| {
    "name": "Wag",
    "type": "Dog",
    "score": 
    [
        9,
        7,
        8
    ]
} |
+---------------------------------------+

Null Argument

If any argument is NULL, the result is NULL:

SELECT 
    JSON_MERGE_PATCH('{"a":1}', null) AS a,
    JSON_MERGE_PATCH(null, '{"a":1}') AS b,
    JSON_MERGE_PATCH(null, null) AS c;

Result:

+------+------+------+
| a    | b    | c    |
+------+------+------+
| NULL | NULL | NULL |
+------+------+------+

Incorrect Parameter Count

Calling the function without any arguments results in an error:

SELECT JSON_MERGE_PATCH();

Result:

ERROR 1582 (42000): Incorrect parameter count in the call to native function 'JSON_MERGE_PATCH'

It’s the same when you provide just one argument:

SELECT JSON_MERGE_PATCH('{"a":1}');

Result:

ERROR 1582 (42000): Incorrect parameter count in the call to native function 'JSON_MERGE_PATCH'