mysql中json_extract的使用方法

mysql中json_extract的使用方法

前言

• mysql5.7版本开始支持JSON类型字段，本文详细介绍json_extract函数如何获取mysql中的JSON类型数据
• json_extract可以完全简写为 ->
• json_unquote(json_extract())可以完全简写为 ->>
• 下面介绍中大部分会利用简写

创建示例表

• 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"}]');
  
  

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

基本语法

• 获取JSON对象中某个key对应的value值

• json_extract函数中，第一个参数content表示json数据，第二个参数为json路径，其中$表示该json数据本身，$.name就表示获取json中key为name的value值

• 可以利用 -> 表达式来代替json_extract

• 若获取的val本身为字符串，那么获取的val会被引号包起来，比如"tom"，这种数据被解析到程序对象中时，可能会被转义为\“tom\”。为了解决这个问题了，可以在外面再包上一层json_unquote函数，或者使用 ->> 代替->

• content:
  {“age”: 18, “name”: “tom”, “score”: [100, 90, 87], “address”: {“city”: “长沙”, “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                |
  +--------------------+
  
  

• 获取JSON数组中某个元素

• json_extract函数中，第一个参数content表示json数据，第二个参数为json路径，其中$表示该json数据本身，$[i]表示获取该json数组索引为i的元素（索引从0开始）

• 与获取key-val一样，若获取的元素为字符串，默认的方式也会得到双引号包起来的字符，导致程序转义，方法也是利用json_unquote函数，或者使用 ->> 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            |
  +------------------+
  
  

• 获取JSON中的嵌套数据

• 结合前面介绍的两种获取方式，可以获取json数据中的嵌套数据

• content: id=1
  {“age”: 18, “name”: “tom”, “score”: [100, 90, 87], “address”: {“city”: “长沙”, “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                  |
  +---------------------+
  

渐入佳境

• 获取JSON多个路径的数据

• 将会把多个路径的数据组合成数组返回

• content: id=1
  {“age”: 18, “name”: “tom”, “score”: [100, 90, 87], “address”: {“city”: “长沙”, “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"]                                              |
  +----------------------------------------------------------------------+
  

• 路径表达式*的使用

• 将会把多个路径的数据组合成数组返回

• # 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”: “长沙”, “name”: “中央公园”}}
  

• # 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"]            |
  +-----------------------------+
  

• 返回NULL值

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

• 寻找的JSON路径都不存在

• # 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                            |
  +---------------------------------+
  

• 路径中有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                               |
  +------------------------------------+
  
  

• 返回错误

• 若第一个参数不是JSON类型的数据，则返回错误

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

• 若路径表达式不规范，则返回错误

• 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.
  

使用场景

• JSON_EXTRACT函数通常用于要获取JSON中某个特定的数据或者要根据它作为判断条件时使用