如何使用 MariaDB 从 JSON 数组中的多个对象中提取值？

Question

In MariaDB 10.2.19, I have a table named forms with a column template which always contains a JSON array of objects. Some of these objects will have properties I want to return: name (should always be present), rule , and parameters . How can I return just these three properties from the entire array, but only for objects on which rule is present?

A sample array (formatted for easier viewing):

[{
    "label": "Employed?",
    "class": "select",
    "name": "employed",
    "parameters": "Yes",
    "rule": "in"
},
{
    "label": "Breed of dog?",
    "class": "select",
    "name": "breed",
    "parameters": "spaniel, collie, mix",
    "rule": "in"
},
{
    "label": "Number",
    "class": "text",
    "name": "breed"
}]

Answer 1

If you are using MySQL 8.0.4 or later one way is using JSON_TABLE :

mysql> SELECT * FROM foo;
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| data                                                                                                                                                                                                                                                                          |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| [{"name": "employed", "rule": "in", "class": "select", "label": "Employed?", "parameters": "Yes"}, {"name": "breed", "rule": "in", "class": "select", "label": "Breed of dog?", "parameters": "spaniel, collie, mix"}, {"name": "breed", "class": "text", "label": "Number"}] |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0,00 sec)

mysql> SELECT name, parameters 
         FROM foo,
              JSON_TABLE (
                  foo.data, 
                  "$[*]" COLUMNS (
                      name VARCHAR(100) PATH "$.name",
                      rule VARCHAR(100) PATH "$.rule",
                      parameters VARCHAR(100) PATH "$.parameters")
              ) AS t
       WHERE rule IS NOT NULL;
+----------+----------------------+
| name     | parameters           |
+----------+----------------------+
| employed | Yes                  |
| breed    | spaniel, collie, mix |
+----------+----------------------+
2 rows in set (0,00 sec)

https://dev.mysql.com/doc/refman/8.0/en/json-table-functions.html

Answer 2

You can accomplish this by using a dedicated numbers_table table:

SELECT
    JSON_VALUE(f.template,CONCAT('$[',n.number,'].name')) AS `name`,
    JSON_VALUE(f.template,CONCAT('$[',n.number,'].rule')) AS `rule`,
    JSON_VALUE(f.template,CONCAT('$[',n.number,'].parameters')) AS `parameters`
FROM forms AS f
JOIN numbers_table AS n
WHERE
    n.number < JSON_LENGTH(f.template) AND
    JSON_VALUE(f.template,CONCAT('$[',n.number,'].rule')) IS NOT NULL;

The numbers_table table contains a single column called number which starts at 0 and can be as long as your use cases require (I have 1000 values 0 to 999 ). It is very useful for extracting each element of a JSON_ARRAY field into its own row.

The first WHERE condition makes sure we only use as many numbers as there are elements in the JSON_ARRAY ( template in your case).

The second WHERE condition eliminates the ones that don't have a rule as per your use case.