mysql中的json_extract怎麼使用

一、前言

mysql5.7版本開始支援JSON類型欄位
json_extract可以完全簡寫為->
json_unquote(json_extract())可以完全簡寫為->>
下面介紹大部分會利用簡寫

#二、創建範例表格

CREATE TABLE `test_json` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `content` json DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4;

# 插入两条测试用的记录
INSERT INTO `test_json` (`content`) VALUES ('{\"name\":\"tom\",\"age\":18,\"score\":[100,90,87],\"address\":{\"province\":\"湖南\",\"city\":\"长沙\"}}');
INSERT INTO `test_json` (`content`) VALUES ('[1, "apple", "red", {"age": 18, "name": "tom"}]');

# #{“age”: 18, “name”: “tom”, “score”: [100, 90, 87], “address”: {“city”: “長沙”, “province”: “湖南”}} 2[1, “apple”, “red”, {“age”: 18, “name”: “tom”}]

id	content
1

三、基本語法

- 取得JSON物件中某個key對應的value值

• json_extract函數中，第一個參數content表示json數據，第二個參數為json路徑，其中$表示該json資料本身，$.name就表示取得json中key為name的value值

• 可以利用-> 表達式來代替json_extract

• 若獲取的val本身為字串，那麼獲取的val會被引號包起來，例如"tom"，這種資料被解析到程式物件中時，可能會被轉義為\“tom\”。為了解決這個問題了，可以在外面再包上一層json_unquote函數，或者使用->> 代替->

content:

{“ age”: 18, “name”: “tom”, “score”: [100, 90, 87], “address”: {“city”: “長沙”, “province”: “湖南”}}

# 得到"tom"
select json_extract(content,'$.name') from test_json where id = 1;
# 简写方式：字段名->表达式等价于json_extract(字段名,表达式)
select content->'$.name' from test_json where id = 1;
# 结果:
+--------------------------------+
| json_extract(content,'$.name') |
+--------------------------------+
| "tom"                          |
+--------------------------------+
+-------------------+
| content->'$.name' |
+-------------------+
| "tom"             |
+-------------------+

# 解除双引号，得到tom
select json_unquote(json_extract(content,'$.name')) from test_json where id = 1;
# 简写方式：字段名->>表达式等价于json_unquote(json_extract(字段名,表达式))
select content->>'$.name' from test_json where id = 1;
# 结果:
+----------------------------------------------+
| json_unquote(json_extract(content,'$.name')) |
+----------------------------------------------+
| tom                                          |
+----------------------------------------------+
+--------------------+
| content->>'$.name' |
+--------------------+
| tom                |
+--------------------+

- 取得JSON數組中某個元素

• json_extract函數中，第一個參數content表示json數據，第二個參數為json路徑，其中$表示該json資料本身，$[i]表示取得該json陣列索引為i的元素（索引從0開始）

• 與取得key-val一樣，若取得的元素為字串，預設的方式也會得到雙引號包起來的字符，導致程式轉義，方法也是利用json_unquote函數，或使用->> 代替->

content:

[1, “apple”, “red”, {“age”: 18, “name”: “tom”}]

# 得到"apple"
select json_extract(content,'$[1]') from test_json where id = 2;
# 简写，效果同上
select content->'$[1]' from test_json where id = 2;
# 结果:
+------------------------------+
| json_extract(content,'$[1]') |
+------------------------------+
| "apple"                      |
+------------------------------+
+-----------------+
| content->'$[1]' |
+-----------------+
| "apple"         |
+-----------------+

# 解除双引号，得到apple 
select json_unquote(json_extract(content,'$[1]')) from test_json where id = 2;
# 简写，效果同上
select content->>'$[1]' from test_json where id = 2;
# 结果:
+--------------------------------------------+
| json_unquote(json_extract(content,'$[1]')) |
+--------------------------------------------+
| apple                                      |
+--------------------------------------------+
+------------------+
| content->>'$[1]' |
+------------------+
| apple            |
+------------------+

- 取得JSON中的巢狀資料

結合前面介紹的兩種取得方式，可以取得json資料中的巢狀資料

content: id=1

{“age”: 18 , “name”: “tom”, “score”: [100, 90, 87], “address”: {“city”: “長沙”, “province”: “湖南”}}
content: id= 2
[1, “apple”, “red”, {“age”: 18, “name”: “tom”}]

# 得到:87
select content->'$.score[2]' from test_json where id = 1;
# 结果:
+-----------------------+
| content->'$.score[2]' |
+-----------------------+
| 87                    |
+-----------------------+

# 得到:18
select content->'$[3].age' from test_json where id = 2;
# 结果:
+---------------------+
| content->'$[3].age' |
+---------------------+
| 18                  |
+---------------------+

四、漸入佳境

#- 取得JSON多個路徑的資料

將會將多個路徑的資料組合成陣列傳回

content: id=1

{「age」: 18, “name”: “tom”, “score”: [100, 90, 87], “address”: {“city”: “長沙”, “province”: “湖南”}}

select json_extract(content,'$.age','$.score') from test_json where id = 1;
# 结果:
+-----------------------------------------+
| json_extract(content,'$.age','$.score') |
+-----------------------------------------+
| [18, [100, 90, 87]]                     |
+-----------------------------------------+

select json_extract(content,'$.name','$.address.province','$.address.city') from test_json where id = 1;
# 结果:
+----------------------------------------------------------------------+
| json_extract(content,'$.name','$.address.province','$.address.city') |
+----------------------------------------------------------------------+
| ["tom", "湖南", "长沙"]                                              |
+----------------------------------------------------------------------+

- 路徑表達式*的使用

將會將多個路徑的資料組合成陣列傳回

# 先插入一条用于测试的数据
INSERT INTO `test_json` (`id`,`content`) VALUES(3,'{"name":"tom","address":{"name":"中央公园","city":"长沙"},"class":{"id":3,"name":"一年三班"},"friend":[{"age":20,"name":"marry"},{"age":21,"name":"Bob"}]}')

content: id=3

{「name ”: “tom”, “class”: {“id”: 3, “name”: “一年三班”}, “friend”: [{“age”: 20, “name”: “marry”}, {“age”: 21, “name”: “Bob”}], “address”: {“city”: “長沙”, “name”: “中央公園”}}

# 获取所有二级嵌套中key=name的值
# 由于friend的二级嵌套是一个数组,所以.name获取不到其中的所有name值
select content->'$.*.name' from test_json where id = 3;
+----------------------------------+
| content->'$.*.name'              |
+----------------------------------+
| ["一年三班", "中央公园"]         |
+----------------------------------+```

# 获取所有key为name值的数据，包括任何嵌套内的name
select content->'$**.name' from test_json where id = 3;
+---------------------------------------------------------+
| content->'$**.name'                                     |
+---------------------------------------------------------+
| ["tom", "一年三班", "marry", "Bob", "中央公园"]         |
+---------------------------------------------------------+

# 获取数组中所有的name值
select content->'$.friend[*].name' from test_json where id = 3;
+-----------------------------+
| content->'$.friend[*].name' |
+-----------------------------+
| ["marry", "Bob"]            |
+-----------------------------+

- 傳回NULL值

content: id=1

{“age”: 18, “name”: “tom”, “score”: [100, 90, 87], "address ”: {“city”: “長沙”, “province”: “湖南”}}

#尋找的JSON路徑都不存在

# age路径不存在，返回NULL
# 若有多个路径，只要有一个路径存在则不会返回NULL
select json_extract(content,'$.price') from test_json where id = 1;
+---------------------------------+
| json_extract(content,'$.price') |
+---------------------------------+
| NULL                            |
+---------------------------------+

路徑中有NULL

#

# 存在任意路径为NULL则返回NULL
select json_extract(content,'$.age',NULL) from test_json where id = 1;
+------------------------------------+
| json_extract(content,'$.age',NULL) |
+------------------------------------+
| NULL                               |
+------------------------------------+

- 回傳錯誤

若第一個參數不是JSON類型的數據，則回傳錯誤

select json_extract('{1,2]',$[0])

若路徑表達式不規範，則傳回錯誤

select content->'$age' from test_json where id = 1;
# 结果:
ERROR 3143 (42000): Invalid JSON path expression. The error is around character position 1.

五、使用場景

JSON_EXTRACT函數通常用於要取得JSON中某個特定的資料或要根據它作為判斷條件時使用

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