How to use json_extract in mysql

##1. Foreword

mysql5.7 version begins to support JSON type fields

json_extract can be completely abbreviated as
-> json_unquote(json_extract()) can be completely abbreviated as
->>Most of the following introductions will use the abbreviation

2. Create Example table

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"}]');

idcontent1{"age": 18, "name": "tom", "score": [100, 90, 87], "address": {"city": "Changsha", "province": "Hunan"}} 2[1, “apple”, “red”, {“age”: 18, “name”: “tom”}]3. Basic syntax

- Obtain the value corresponding to a key in the JSON object

• In the json_extract function, the first One parameter content represents json data, and the second parameter is the json path, where $ represents the json data itself, and $.name represents obtaining the value with the key name in json

• You can use -> expression instead of json_extract

• If the obtained val itself is a string, then the obtained val will be wrapped in quotation marks, such as "tom", this kind of data is When parsed into a program object, it may be escaped as \"tom\". In order to solve this problem, you can wrap another layer of json_unquote function outside, or use ->> instead of ->

content:

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

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

- Get an element in the JSON array

• In the json_extract function, the first parameter content represents json data, and the second parameter is the json path, where $ Represents the json data itself, $[i] indicates obtaining the element with index i of the json array (the index starts from 0)

• is the same as obtaining key-val, if the obtained element is For strings, the default method will also get characters enclosed in double quotes, causing the program to escape. The method is also to use the json_unquote function, or use ->> instead of ->

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

- Get JSON Nested data

Combined with the two acquisition methods introduced earlier, you can obtain the nested data in json data

content: id=1

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

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                  |
+---------------------+

4. Getting better

- Obtain JSON data of multiple paths

Will combine the data of multiple paths into an array and return

content: id=1

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

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"]                                              |
+----------------------------------------------------------------------+

- The use of path expression *

will combine the data of multiple paths into an array and return

# 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": "Class Three a Year"}, "friend": [{"age": 20, "name": "marry"}, {"age": 21, "name": "Bob"}], "address": {"city": "Changsha", "name": "Central Park"}}

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

- Return NULL value

content: id=1

{"age": 18, "name": "tom", "score": [100, 90, 87], "address" ": {"city": "Changsha", "province": "Hunan"}}

The JSON path you are looking for does not exist

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

There are NULL

in the path

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

- Return error

If the first parameter is not JSON type data, return error

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

If the path expression is not standardized, return error

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.

5. Usage scenarios

The JSON_EXTRACT function is usually used when you want to obtain a specific data in JSON or use it as a judgment condition

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