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

JSON_MERGE_PRESERVE() is a synonym for JSON_MERGE(), which has been deprecated. To avoid future issues, you should use the JSON_MERGE_PATCH() function instead. The JSON_MERGE_PATCH() function is an RFC 7396-compliant replacement for JSON_MERGE().

Syntax

The syntax goes like this:

JSON_MERGE_PRESERVE(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_PRESERVE(
    '{"name":"Wag"}', 
    '{"type":"Dog"}'
    ) AS Result;

Result:

+--------------------------------+
| Result                         |
+--------------------------------+
| {"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_PRESERVE(
    '{ "name" : "Wag" }', 
    '{ "type" : "Dog" }',
    '{ "score" : [ 9, 7, 8 ] }'
    ) AS Result;

Result:

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

Arrays

One difference between JSON_MERGE_PRESERVE() and JSON_MERGE_PATCH() is that JSON_MERGE_PRESERVE() merges arrays (JSON_MERGE_PATCH() does not):

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

Result:

+--------------------+
| Result             |
+--------------------+
| [1, 2, 3, 4, 5, 6] |
+--------------------+

Attempting this with JSON_MERGE_PATCH() results in only the second array being returned.

In case you’re wondering, yes JSON_MERGE() also merges arrays.

Null Argument

If any argument is NULL, the result is NULL:

SELECT 
    JSON_MERGE_PRESERVE('{"a":1}', null) AS a,
    JSON_MERGE_PRESERVE(null, '{"a":1}') AS b,
    JSON_MERGE_PRESERVE(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_PRESERVE();

Result:

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

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

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

Result:

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