mysql json数据类型

一、概述

mysql从5.7后引入了json数据类型以及json函数，可以有效的访问json格式的数据。json数据类型相对于字符串，具有以下优点：

1）对于json列数据提供自动校验json格式，错误格式会提示错误；

2）优化存储类型。数据以二进制方式保存，读取效率快；

3）允许通过键值或者数据索引查找对象，无需把整个数据读取出来；

除此之外，json还有以下特点：

1）json存储空间大致于longblob或longtext差不多；

2）mysql8.0.13之后，json允许默认值为null；

3）json列不能设置索引，可通过json中的键值设置索引来提高查询效率；

4）json中null、true、false必须使用小写，

二、JSON函数介绍

1、创建JSON函数

JSON_ARRAY([val[, val] ...])：

每一个val值作为json数据中的元素

mysql> select JSON_ARRAY(1,'2',now());
+----------------------------------------+
| JSON_ARRAY(1,'2',now())                |
+----------------------------------------+
| [1, "2", "2021-10-26 12:00:03.000000"] |
+----------------------------------------+
1 row in set (0.00 sec)

 JSON_OBJECT([key, val[, key, val] ...])：

格式为：key，value，key，value...

mysql> select JSON_OBJECT('name','main','age',18,'birthday',now());
+-----------------------------------------------------------------------+
| JSON_OBJECT('name','main','age',18,'birthday',now())                  |
+-----------------------------------------------------------------------+
| {"age": 18, "name": "main", "birthday": "2021-10-26 12:06:31.000000"} |
+-----------------------------------------------------------------------+
1 row in set (0.00 sec)

创建含有json字段的表结构：

mysql> create table json_test(id int, content json default null);
Query OK, 0 rows affected (0.63 sec)

mysql> insert into json_test values (10000, '{"name":"test","age":18,"body":{"height":180,"weight":70,"friends":["Tom","Lucy","Lili"]}}');
Query OK, 1 row affected (0.26 sec)

2、查询JSON函数

JSON_CONTAINS(target, candidate[, path])

通过返回0或1，判断目标对象target是否包含candidate元素，如果查询包含path路径，则在指定的路径上面查找。

mysql> select JSON_CONTAINS('[1,2,"3",4]','1');
+----------------------------------+
| JSON_CONTAINS('[1,2,"3",4]','1') |
+----------------------------------+
|                                1 |
+----------------------------------+
1 row in set (0.00 sec)
mysql> select JSON_CONTAINS('[1,2,"3",4]','3');
+----------------------------------+
| JSON_CONTAINS('[1,2,"3",4]','3') |
+----------------------------------+
|                                0 |
+----------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_CONTAINS('[1,2,"3",4]','"3"');
+------------------------------------+
| JSON_CONTAINS('[1,2,"3",4]','"3"') |
+------------------------------------+
|                                  1 |
+------------------------------------+
1 row in set (0.00 sec)
mysql> select JSON_CONTAINS('{"name":"test","age":0}','0','$.age');
+------------------------------------------------------+
| JSON_CONTAINS('{"name":"test","age":0}','0','$.age') |
+------------------------------------------------------+
|                                                    1 |
+------------------------------------------------------+
1 row in set (0.00 sec)

注意：JSON格式的数据都是字符串，单个字符也要加引号，比如数字 '0'，如果是表示字符串，'"0"'，方法中的参数必须符合json类型，否则报错。

JSON_CONTAINS_PATH(target, one_or_all, path[, path] ...)

通过返回0或1，判断在目标对象target中，指定路径是否包含数据，one_or_all只能是one或者all，如果是one表示多个path下有一个有数据返回1，如果是all，指定的path下必须都有值才能返回1。

mysql> select JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','one','$.a','$.c');
+----------------------------------------------------------------------------+
| JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','one','$.a','$.c') |
+----------------------------------------------------------------------------+
|                                                                          1 |
+----------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','ONE','$.a','$.c');
+----------------------------------------------------------------------------+
| JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','ONE','$.a','$.c') |
+----------------------------------------------------------------------------+
|                                                                          1 |
+----------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','all','$.a','$.c');
+----------------------------------------------------------------------------+
| JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','all','$.a','$.c') |
+----------------------------------------------------------------------------+
|                                                                          0 |
+----------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','all','$.a','$.b');
+----------------------------------------------------------------------------+
| JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','all','$.a','$.b') |
+----------------------------------------------------------------------------+
|                                                                          1 |
+----------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','all','$.a','$.b.b2');
+-------------------------------------------------------------------------------+
| JSON_CONTAINS_PATH('{"a":1,"b":{"b1:":3,"b2":4},"d":0}','all','$.a','$.b.b2') |
+-------------------------------------------------------------------------------+
|                                                                             1 |
+-------------------------------------------------------------------------------+
1 row in set (0.00 sec)

JSON_EXTRACT(target, path[, path] ...)

返回所查询path路径对应的value值，如果查询多个path，返回值自动组装成数组

mysql> select JSON_EXTRACT('[1,2,[4,5],6,7]','$[2]');
+----------------------------------------+
| JSON_EXTRACT('[1,2,[4,5],6,7]','$[2]') |
+----------------------------------------+
| [4, 5]                                 |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_EXTRACT('[1,2,[4,5],6,7]','$[2][1]');
+-------------------------------------------+
| JSON_EXTRACT('[1,2,[4,5],6,7]','$[2][1]') |
+-------------------------------------------+
| 5                                         |
+-------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_EXTRACT('[1,2,[4,5],6,7]','$[2][2]');
+-------------------------------------------+
| JSON_EXTRACT('[1,2,[4,5],6,7]','$[2][2]') |
+-------------------------------------------+
| NULL                                      |
+-------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_EXTRACT('[1,2,[4,5],6,7]','$[2]','$[3]');
+-----------------------------------------------+
| JSON_EXTRACT('[1,2,[4,5],6,7]','$[2]','$[3]') |
+-----------------------------------------------+
| [[4, 5], 6]                                   |
+-----------------------------------------------+
1 row in set (0.01 sec)
mysql> select JSON_EXTRACT('{"a":1,"b":{"b1:":3,"b2":4},"d":0}', '$.b', '$.d');
+------------------------------------------------------------------+
| JSON_EXTRACT('{"a":1,"b":{"b1:":3,"b2":4},"d":0}', '$.b', '$.d') |
+------------------------------------------------------------------+
| [{"b2": 4, "b1:": 3}, 0]                                         |
+------------------------------------------------------------------+
1 row in set (0.00 sec)

column->path

->是JSON_EXTRACT(target, path)的缩减版(只有一个path场景)，->可用于日常SQL表达式中，比如update、delete、select等。

+-------+--------------------------------------------------------------------------------------------------------+
| id    | content                                                                                                |
+-------+--------------------------------------------------------------------------------------------------------+
| 10000 | {"name": "test"}                                                                                       |
| 10000 | {"age": 18, "body": "{}", "name": "test"}                                                              |
| 10000 | {"age": 18, "body": {"age": 19}, "name": "test"}                                                       |
| 10000 | {"age": 18, "body": {"height": 180, "weight": 70, "friends": ["Tom", "Lucy", "Lili"]}, "name": "test"} |
|     1 | {"body": "{\"weight\":70}", "name": "test"}                                                            |
|     1 | {"body": "{weight:70}", "name": "test"}                                                                |
+-------+--------------------------------------------------------------------------------------------------------+

mysql> select content->'$.name' from json_test where content->'$.body.age' = 19;
+-------------------+
| content->'$.name' |
+-------------------+
| "test"            |
+-------------------+
1 row in set (0.00 sec)

column->>path

column->path返回的包含引号""，column->>path返回的结果自动去掉""，与下面几种方式返回功能一样：

JSON_UNQUOTE( JSON_EXTRACT(column, path) )
JSON_UNQUOTE(column -> path)
column->>path

mysql> select content->>'$.name' from json_test where content->'$.body.age' = 19;
+--------------------+
| content->>'$.name' |
+--------------------+
| test               |
+--------------------+
1 row in set (0.00 sec)

mysql> select JSON_UNQUOTE(content->'$.name') from json_test where content->'$.body.age' = 19;
+---------------------------------+
| JSON_UNQUOTE(content->'$.name') |
+---------------------------------+
| test                            |
+---------------------------------+
1 row in set (0.01 sec)

mysql> select JSON_UNQUOTE(JSON_EXTRACT(content, '$.name')) from json_test where content->'$.body.age' = 19;
+-----------------------------------------------+
| JSON_UNQUOTE(JSON_EXTRACT(content, '$.name')) |
+-----------------------------------------------+
| test                                          |
+-----------------------------------------------+
1 row in set (0.00 sec)

JSON_KEYS(target[, path])

如果path为空，将target对象中第一层key值组装成数组返回，如果path不为空，在指定path的路径下返回key值组成后的数组

mysql> select JSON_KEYS('{"age": 18, "body": {"height": 180, "weight": 70, "friends": ["Tom", "Lucy", "Lili"]}, "name": "test"}');
+---------------------------------------------------------------------------------------------------------------------+
| JSON_KEYS('{"age": 18, "body": {"height": 180, "weight": 70, "friends": ["Tom", "Lucy", "Lili"]}, "name": "test"}') |
+---------------------------------------------------------------------------------------------------------------------+
| ["age", "body", "name"]                                                                                             |
+---------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_KEYS('{"age": 18, "body": {"height": 180, "weight": 70, "friends": ["Tom", "Lucy", "Lili"]}, "name": "test"}', '$.body');
+-------------------------------------------------------------------------------------------------------------------------------+
| JSON_KEYS('{"age": 18, "body": {"height": 180, "weight": 70, "friends": ["Tom", "Lucy", "Lili"]}, "name": "test"}', '$.body') |
+-------------------------------------------------------------------------------------------------------------------------------+
| ["height", "weight", "friends"]                                                                                               |
+-------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

2、修改JSON函数

JSON_ARRAY_APPEND(json_doc, path, val[, path, val] ...)

在指定路径上追加元素

mysql> select JSON_ARRAY_APPEND('{"name":"test"}','$.name','{"age":20}');
+------------------------------------------------------------+
| JSON_ARRAY_APPEND('{"name":"test"}','$.name','{"age":20}') |
+------------------------------------------------------------+
| {"name": ["test", "{\"age\":20}"]}                         |
+------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_ARRAY_APPEND('[1,[2,3],4,5]','$',0,'$[4]',6);
+---------------------------------------------------+
| JSON_ARRAY_APPEND('[1,[2,3],4,5]','$',0,'$[4]',6) |
+---------------------------------------------------+
| [1, [2, 3], 4, 5, [0, 6]]                         |
+---------------------------------------------------+
1 row in set (0.00 sec)

JSON_ARRAY_INSERT(json_doc, path, val[, path, val] ...)

在指定的path上添加元素，相对于JSON_ARRAY_APPEND，INSERT是新加元素，不会和之前存在的元素组成新的数组

mysql> select JSON_ARRAY_INSERT('{"name":[1,2]}','$.name[2]',1);
+---------------------------------------------------+
| JSON_ARRAY_INSERT('{"name":[1,2]}','$.name[2]',1) |
+---------------------------------------------------+
| {"name": [1, 2, 1]}                               |
+---------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_ARRAY_INSERT('[1,3,[5,7],9,11]','$[1]',2);
+------------------------------------------------+
| JSON_ARRAY_INSERT('[1,3,[5,7],9,11]','$[1]',2) |
+------------------------------------------------+
| [1, 2, 3, [5, 7], 9, 11]                       |
+------------------------------------------------+
1 row in set (0.00 sec)

选择的元素必须是数组，否则无效：

mysql> select JSON_ARRAY_INSERT('[1,3,[5,7],{"a": [0,10]},11]','$[3][0]',2);
+---------------------------------------------------------------+
| JSON_ARRAY_INSERT('[1,3,[5,7],{"a": [0,10]},11]','$[3][0]',2) |
+---------------------------------------------------------------+
| [1, 3, [5, 7], {"a": [0, 10]}, 11]                            |
+---------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_ARRAY_INSERT('[1,3,[5,7],{"a": [0,10]},11]','$[3].a[0]',2);
+-----------------------------------------------------------------+
| JSON_ARRAY_INSERT('[1,3,[5,7],{"a": [0,10]},11]','$[3].a[0]',2) |
+-----------------------------------------------------------------+
| [1, 3, [5, 7], {"a": [2, 0, 10]}, 11]                           |
+-----------------------------------------------------------------+
1 row in set (0.00 sec)

JSON_INSERT(json_doc, path, val[, path, val] ...)

向JSON数据中添加元素

mysql> select JSON_INSERT('{"name":"test","age":18,"body":{"weight":70}}','$.name',"new-test",'$.body.height','180');
+--------------------------------------------------------------------------------------------------------+
| JSON_INSERT('{"name":"test","age":18,"body":{"weight":70}}','$.name',"new-test",'$.body.height','180') |
+--------------------------------------------------------------------------------------------------------+
| {"age": 18, "body": {"height": "180", "weight": 70}, "name": "test"}                                   |
+--------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

因为$.name已经存在，所以插入无效。

JSON_REPLACE(json_doc, path, val[, path, val] ...)

替换JSON数据中原有数据，如果没有，则不更新

mysql> select JSON_REPLACE('{"name":"test","age":18,"body":{"weight":70}}','$.name',"new-test",'$.body.height','180');
+---------------------------------------------------------------------------------------------------------+
| JSON_REPLACE('{"name":"test","age":18,"body":{"weight":70}}','$.name',"new-test",'$.body.height','180') |
+---------------------------------------------------------------------------------------------------------+
| {"age": 18, "body": {"weight": 70}, "name": "new-test"}                                                 |
+---------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

JSON_SET(json_doc, path, val[, path, val] ...)

如果JSON数据中存在path，则更新数据，如果不存在，插入数据

mysql> select JSON_SET('{"name":"test","age":18,"body":{"weight":70}}','$.name',"new-test",'$.body.height','180');
+-----------------------------------------------------------------------------------------------------+
| JSON_SET('{"name":"test","age":18,"body":{"weight":70}}','$.name',"new-test",'$.body.height','180') |
+-----------------------------------------------------------------------------------------------------+
| {"age": 18, "body": {"height": "180", "weight": 70}, "name": "new-test"}                            |
+-----------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

JSON_MERGE_PATCH(json_doc, json_doc[, json_doc] ...)

将多个JSON数据合并：

1）如果第一个JSON不是对象，合并的结果为第二个对象

mysql> select JSON_MERGE_PATCH('1','{"name":"test"}');
+-----------------------------------------+
| JSON_MERGE_PATCH('1','{"name":"test"}') |
+-----------------------------------------+
| {"name": "test"}                        |
+-----------------------------------------+
1 row in set (0.00 sec)

2）如果第二个JSON不是对象，则合并的结果为第二个元素

mysql> select JSON_MERGE_PATCH('{"name":"test"}','1');
+-----------------------------------------+
| JSON_MERGE_PATCH('{"name":"test"}','1') |
+-----------------------------------------+
| 1                                       |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_MERGE_PATCH('1','{"name":"test"}', '1', '{"b":1}');
+---------------------------------------------------------+
| JSON_MERGE_PATCH('1','{"name":"test"}', '1', '{"b":1}') |
+---------------------------------------------------------+
| {"b": 1}                                                |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_MERGE_PATCH('{"name":"test"}', '1', '{"b":1}');
+-----------------------------------------------------+
| JSON_MERGE_PATCH('{"name":"test"}', '1', '{"b":1}') |
+-----------------------------------------------------+
| {"b": 1}                                            |
+-----------------------------------------------------+
1 row in set (0.00 sec)

3）如果都是对象，有重复的key值，后面的value会覆盖前面的值

mysql> select JSON_MERGE_PATCH('{"name":"test"}','{"a":1}');
+-----------------------------------------------+
| JSON_MERGE_PATCH('{"name":"test"}','{"a":1}') |
+-----------------------------------------------+
| {"a": 1, "name": "test"}                      |
+-----------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_MERGE_PATCH('{"name":"test","age":20,"body":{"weight":180}}','{"a":0,"age":18,"body":{"height":180,"weight":70}}');
+-------------------------------------------------------------------------------------------------------------------------+
| JSON_MERGE_PATCH('{"name":"test","age":20,"body":{"weight":180}}','{"a":0,"age":18,"body":{"height":180,"weight":70}}') |
+-------------------------------------------------------------------------------------------------------------------------+
| {"a": 0, "age": 18, "body": {"height": 180, "weight": 70}, "name": "test"}                                              |
+-------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

4）合并后，会将value为null的过滤掉

mysql> select JSON_MERGE_PATCH('{"name":"test","age":20,"body":{"weight":180}}', '{"NAME":"new","salary":null}');
+----------------------------------------------------------------------------------------------------+
| JSON_MERGE_PATCH('{"name":"test","age":20,"body":{"weight":180}}', '{"NAME":"new","salary":null}') |
+----------------------------------------------------------------------------------------------------+
| {"age": 20, "NAME": "new", "body": {"weight": 180}, "name": "test"}                                |
+----------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

根据第一和第四可以通过使用常量+JSON对象过滤为null的键值

mysql> select JSON_MERGE_PATCH('1','{"name":"test","salary":null}');
+-------------------------------------------------------+
| JSON_MERGE_PATCH('1','{"name":"test","salary":null}') |
+-------------------------------------------------------+
| {"name": "test"}                                      |
+-------------------------------------------------------+
1 row in set (0.00 sec)

JSON_MERGE_PRESERVE(json_doc, json_doc[, json_doc] ...)

相对于JSON_MERGE_PATCH，如果有重复的key值，会组成新的数组元素

mysql> select JSON_MERGE_PRESERVE('{"name":"test","age":20}','{"a":0,"age":18}');
+--------------------------------------------------------------------+
| JSON_MERGE_PRESERVE('{"name":"test","age":20}','{"a":0,"age":18}') |
+--------------------------------------------------------------------+
| {"a": 0, "age": [20, 18], "name": "test"}                          |
+--------------------------------------------------------------------+
1 row in set (0.00 sec)

两个数组或常量会生成新的数组

mysql> select JSON_MERGE_PRESERVE('[1,2]','["a","b"]');
+------------------------------------------+
| JSON_MERGE_PRESERVE('[1,2]','["a","b"]') |
+------------------------------------------+
| [1, 2, "a", "b"]                         |
+------------------------------------------+
1 row in set (0.00 sec)
mysql> select JSON_MERGE_PRESERVE('1','2','"a"');
+------------------------------------+
| JSON_MERGE_PRESERVE('1','2','"a"') |
+------------------------------------+
| [1, 2, "a"]                        |
+------------------------------------+
1 row in set (0.00 sec)

两个对象会生成新的对象，有相关key值的不会被覆盖

mysql> select JSON_MERGE_PRESERVE('{"name":"test","age":20}','{"a":0,"age":18}');
+--------------------------------------------------------------------+
| JSON_MERGE_PRESERVE('{"name":"test","age":20}','{"a":0,"age":18}') |
+--------------------------------------------------------------------+
| {"a": 0, "age": [20, 18], "name": "test"}                          |
+--------------------------------------------------------------------+
1 row in set (0.00 sec)

一个对象和一个数组会生成新的数组

mysql> select JSON_MERGE_PRESERVE('{"name":"test","age":20}','[1,2,"a"]');
+-------------------------------------------------------------+
| JSON_MERGE_PRESERVE('{"name":"test","age":20}','[1,2,"a"]') |
+-------------------------------------------------------------+
| [{"age": 20, "name": "test"}, 1, 2, "a"]                    |
+-------------------------------------------------------------+
1 row in set (0.00 sec)

JSON_REMOVE(json_doc, path[, path] ...)

将指定path的元素删除

mysql> select JSON_REMOVE('{"name":"test"}','$.name');
+-----------------------------------------+
| JSON_REMOVE('{"name":"test"}','$.name') |
+-----------------------------------------+
| {}                                      |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_REMOVE('{"name":"test","age":18}','$.name');
+--------------------------------------------------+
| JSON_REMOVE('{"name":"test","age":18}','$.name') |
+--------------------------------------------------+
| {"age": 18}                                      |
+--------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_REMOVE('{"name":"test","age":18}','$.body');
+--------------------------------------------------+
| JSON_REMOVE('{"name":"test","age":18}','$.body') |
+--------------------------------------------------+
| {"age": 18, "name": "test"}                      |
+--------------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_REMOVE('[1,2,3]','$[0]');
+-------------------------------+
| JSON_REMOVE('[1,2,3]','$[0]') |
+-------------------------------+
| [2, 3]                        |
+-------------------------------+
1 row in set (0.00 sec)

其他JSON函数可参考官网：MySQL :: MySQL 8.0 Reference Manual :: 12.18 JSON Functions

三、JSON类型其他特性

json数据比较和排序支持=, <, <=, >, >=, <>, !=, and <=>，不支持BETWEEN，IN()，GREATEST()，LEAST()；

JSON值比较有优先级，下面是优先级从高到低，不同层级的优先级，按照优先级高的确定

BLOB
BIT
OPAQUE
DATETIME
TIME
DATE
BOOLEAN
ARRAY
OBJECT
STRING
INTEGER, DOUBLE
NULL

mysql> select JSON_ARRAY(1) > JSON_ARRAY('1');
+---------------------------------+
| JSON_ARRAY(1) > JSON_ARRAY('1') |
+---------------------------------+
|                               0 |
+---------------------------------+
1 row in set (0.00 sec)
mysql> select JSON_ARRAY('a','d') > JSON_ARRAY('a','b');
+-------------------------------------------+
| JSON_ARRAY('a','d') > JSON_ARRAY('a','b') |
+-------------------------------------------+
|                                         1 |
+-------------------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_ARRAY('1','d') > JSON_ARRAY('a','b');
+-------------------------------------------+
| JSON_ARRAY('1','d') > JSON_ARRAY('a','b') |
+-------------------------------------------+
|                                         0 |
+-------------------------------------------+
1 row in set (0.00 sec)

非JSON格式数据与JSON数据转换

other type	CAST(other type AS JSON)	CAST(JSON AS other type)
JSON	不变	不变
utf8字符类型(utf8mb4,utf8,ascii)	字符串被解析为JSON值	JSON被序列化成utf8mb4字符串
其他字符类型	其他字符编码被隐式转换为utf8mb4，并按utf8字符类型进处理	JSON值被序列化为utf8mb4字符串，然后再被转换为其他字符编码。结果可能没有意义。
NULL	结果为JSON类型的null值	不适用

四、JSON索引

json列不支持创建索引，可根据json数据中某个key值创建虚拟列索引，创建方式为：

column_name [ GENERATED ALWAYS ] AS ( ) [ VIRTUAL|STORED ] [ UNIQUE [KEY] ] [ [PRIMARY] KEY ] [ NOT NULL ] [ COMMENT ]

GENERATED ALWAYS：通俗的讲这一列是通过其他列计算所得

mysql 5.7之后，支持Virtual Generated Column和Stored Generated Column两种Generated Column；前者只将Generated Column保存在数据字典中（表的元数据），后者会将Generated Column持久化到磁盘上。默认是Virtual Column。

可以在建表的时候直接创建虚拟列索引，也可以先建虚拟列，然后建索引，跟平常创建索引方式一样。

create table json_test(id int, content json default null, name varchar(64) GENERATED ALWAYS AS (`content`-> '$.name') NOT NULL);

alter table json_test add index idx_name(`name`);

创建索引之前查询走的是全表查询

mysql> desc json_test;
+---------+-------------+------+-----+---------+-------------------+
| Field   | Type        | Null | Key | Default | Extra             |
+---------+-------------+------+-----+---------+-------------------+
| id      | int         | YES  |     | NULL    |                   |
| content | json        | YES  |     | NULL    |                   |
| name    | varchar(64) | NO   |     | NULL    | VIRTUAL GENERATED |
+---------+-------------+------+-----+---------+-------------------+
3 rows in set (0.00 sec)

mysql> explain select * from json_test where content->'$.name' = 'test';
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | json_test | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from json_test where name = 'test';
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | json_test | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

创建索引之后走的是索引查询

mysql> desc json_test;
+---------+-------------+------+-----+---------+-------------------+
| Field   | Type        | Null | Key | Default | Extra             |
+---------+-------------+------+-----+---------+-------------------+
| id      | int         | YES  |     | NULL    |                   |
| content | json        | YES  |     | NULL    |                   |
| name    | varchar(64) | NO   | MUL | NULL    | VIRTUAL GENERATED |
+---------+-------------+------+-----+---------+-------------------+
3 rows in set (0.00 sec)

mysql> explain select * from json_test where content->'$.name' = 'test';
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | json_test | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from json_test where name = 'test';
+----+-------------+------------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table      | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+----------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | json_test | NULL       | ref  | idx_name      | idx_name | 258     | const |    1 |   100.00 | NULL  |
+----+-------------+------------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)