我们都知道oracle存储过程支持为参数设置默认值，这样即使存储过程升级，原来的调用也可以不受影响。但是mysql不支持，mariadb也没有支持（截止10.4也是如此）。但是这一限制会导致升级麻烦重重。虽然如此，我们可以通过mysql 5.7/mariadb 10.2引入的json类型来变通实现。如下所示：

drop function number_stats;
CREATE FUNCTION number_stats(in_numbers JSON)
    RETURNS INTEGER
    NOT DETERMINISTIC
    CONTAINS SQL
    COMMENT 'Accept an array of integers and their median'
BEGIN
    DECLARE v_count INT UNSIGNED
        DEFAULT JSON_LENGTH(in_numbers);
    RETURN JSON_EXTRACT(
        in_numbers,
        CONCAT('$[', FLOOR(v_count / 2), ']')
    );
END;

 

mariadb> select number_stats('[1,2,3,4]');

+---------------------------+
| number_stats('[1,2,3,4]') |
+---------------------------+
|                         3 |
+---------------------------+
1 row in set

mariadb> select JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Monty" }','$.z');
+----------------------------------------------------------------+
| JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Monty" }','$.z') |
+----------------------------------------------------------------+
| Monty                                                          |
+----------------------------------------------------------------+
1 row in set

mariadb> select JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Monty" }','$.x
');
+----------------------------------------------------------------+
| JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Monty" }','$.x') |
+----------------------------------------------------------------+
| NULL                                                           |
+----------------------------------------------------------------+
1 row in set

mariadb> select JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Monty" }','$.y
');
+----------------------------------------------------------------+
| JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Monty" }','$.y') |
+----------------------------------------------------------------+
| [0,1]                                                          |
+----------------------------------------------------------------+
1 row in set

https://federico-razzoli.com/variable-number-of-parameters-and-optional-parameters-in-mysql-mariadb-procedures

https://mariadb.com/kb/en/library/json_extract/