MySQL JSON_OBJECT 而不是 GROUP_CONCAT

Question

我有以下 sql 查询，使用 GROUP_CONCAT 可以正常工作:

SELECT orders.created_at,products.title, o_p.qty AS qty,

    (SELECT GROUP_CONCAT(features.title,"\:", o_p_f.value, features.unit) FROM order_product_features AS o_p_f 
     LEFT JOIN product_features ON product_features.id = o_p_f.product_feature_id
    LEFT JOIN features ON o_p_f.product_feature_id = product_features.id
     AND features.id = product_features.feature_id

     WHERE o_p_f.order_product_id = o_p.id
) AS prop
FROM orders
LEFT JOIN order_products AS o_p ON o_p.order_id = orders.id

LEFT JOIN products ON products.id = o_p.product_id

上面的查询返回结果如下图所示:

现在，我想用 JSON_OBJECT 替换 GROUP_CONCAT 或 换句话说，我想让 prop 字段成为 JSON对象。我尝试了以下方法:

SELECT orders.created_at,products.title, o_p.qty AS qty,

    JSON_OBJECT((SELECT GROUP_CONCAT("title",features.title,"value", o_p_f.value, 'unit',features.unit) FROM order_product_features AS o_p_f 
     LEFT JOIN product_features ON product_features.id = o_p_f.product_feature_id
    LEFT JOIN features ON o_p_f.product_feature_id = product_features.id
     AND features.id = product_features.feature_id

     WHERE o_p_f.order_product_id = o_p.id
)) AS prop
FROM orders
LEFT JOIN order_products AS o_p ON o_p.order_id = orders.id

LEFT JOIN products ON products.id = o_p.product_id

但是，上面的查询返回错误:

1582 - Incorrect parameter count in the call to native function 'JSON_OBJECT'

Answer 1

在 ≥ 5.7.22

JSON_OBJECTAGG(key, value)更有可能是您所追求的。

mysql> SELECT o_id, attribute, value FROM t3;
+------+-----------+-------+
| o_id | attribute | value |
+------+-----------+-------+
|    2 | color     | red   |
|    2 | fabric    | silk  |
|    3 | color     | green |
|    3 | shape     | square|
+------+-----------+-------+
4 rows in set (0.00 sec)

mysql> SELECT o_id, JSON_OBJECTAGG(attribute, value) FROM t3 GROUP BY o_id;
+------+----------------------------------------+
| o_id | JSON_OBJECTAGG(attribute, name)        |
+------+----------------------------------------+
|    2 | {"color": "red", "fabric": "silk"}     |
|    3 | {"color": "green", "shape": "square"}  |
+------+----------------------------------------+
1 row in set (0.00 sec)

但是因为你没有提供你的表结构我不能帮你查询。

看来你对 unit 的处理会给你一些额外的工作，因为聚合函数使用 Key=>Value 并且不会接受第三个参数...

≥5.7

你必须做一些手工:

SELECT 
o_id, 
CONCAT(
    '{', 
    GROUP_CONCAT(
        TRIM(
            LEADING '{' FROM TRIM(
                TRAILING '}' FROM JSON_OBJECT(
                    `attribute`, 
                    `value`
                    )
                )
            )
        ),
    '}'
    ) json 
FROM t3 
GROUP BY o_id

≥5.5

没有任何 JSON 函数:

SELECT 
o_id, 
CONCAT(
    '{', 
    GROUP_CONCAT(
        CONCAT(
            '"',
            `attribute`,
            '":"',
            `value`,
            '"'
            )
        ),
    '}'
    ) json 
FROM t3 
GROUP BY o_id