mysql - 我可以在 MySQL 中获取 json 列的唯一/不同值吗？

Question

这个有点难解释，所以我会举个例子来解释，

假设我有这样的表(标签是 json 列)

+----+-------------------+--------------------+
| id |    occupation     |        tags        |
+----+-------------------+--------------------+
|  1 | Mold Maker        | [Men, Shop, Shoes] |
|  2 | Software Engineer | [Men, Lifestyle]   |
|  3 | Electrician       | [Shop, Lifestyle]  |
|  4 | Software Engineer | [Men, Lifestyle]   |
|  5 | Software Engineer | [Shoes]            |
+----+-------------------+--------------------+

当我想获得职业的唯一值(value)时，我只是像这样查询。

SELECT DISTINCT occupation FROM customers;
或者
SELECT occupation FROM customers GROUP BY occupation;

result
+-------------------+
|    occupation     |
+-------------------+
| Mold Maker        |
| Software Engineer |
| Electrician       |
+-------------------+

我希望标签的唯一值如下所示

+-----------+
|   tags    |
+-----------+
| Men       |
| Shop      |
| Shoes     |
| Lifestyle |
+-----------+

到目前为止，我尝试阅读 MySQL 手册和谷歌中的所有 JSON_* 函数和 JSON_TABLE，但找不到方法，是否有办法获得我想要的结果。

Answer 1

在 MySQL 8.0 中，json 函数 json_table() 可以派上用场:

select distinct tag
from 
    mytable,
    json_table(
        tags,
        "$[*]"
        columns (tag varchar(50) PATH "$")
    ) t
order by tag

在早期版本中，一种解决方案是使用数字表。这假设您事先知道 json 数组中元素的最大数量:

select distinct replace(
    json_extract(tags, concat('$[', nums.n, ']')),
    '"',
    ''
) tag
from 
    (
        select 0 n 
        union all select 1 
        union all select 2 
        union all select 3 
        union all select 4
    ) nums
    inner join mytable t
        on json_extract(t.tags, concat('$[', nums.n, ']')) is not null 
    order by tag

Demo on DB Fiddle

| tag       |
| :-------- |
| Lifestyle |
| Men       |
| Shoes     |
| Shop      |