JSON 代表 JavaScript Object
Notation。JSON是开放的标准格式，由key-value对组成。JSON的主要用于在服务器与web应用之间传输数据。

PostgreSQL提供了两种存储JSON数据的类型：json和jsonb；

jsonb是json的二进制形式。

json格式写入快，但读取慢；
jsonb格式写入慢，但读取快；

常用语法

 // -> 返回json
 select '[{"a":"foo"},{"b":"bar"},{"c":"baz"}]'::json->2 // 输出 {"c":"baz"}
 select '{"a": {"b":"foo"}, "c":{"a": "aaa"}}'::json->'a' // 输出 {"b":"foo"}
 
 // ->> 返回文本
 select '[{"a":"foo"},{"b":"bar"},{"c":"baz"}]'::json->>2 // 输出 {"c":"baz"}
 select '{"a": {"b":"foo"}, "c":{"a": "aaa"}}'::json->>'a' // 输出 {"b":"foo"}

 // #> 获取json子对象
 select '{"a": {"b":{"c": "foo"}}}'::json#> '{a,b}' // 输出 {"c": "foo"}
 select '{"a": {"b":{"c": "foo"}}}'::json#>> '{a,b}' // 输出 {"c": "foo"}

 // @> ———— 判断第一个json是否包含第二个
 select '{"a":1, "b":2}'::jsonb @> '{"b":2}'::jsonb   //输出t
 // <@ ———— 判断第一个json是否在第一个中
 select '{"b":2}'::jsonb <@ '{"a":1, "b":2}'::jsonb   //输出t

常用操作符

实例操作
数据库表(test_pgsql)：

第一条data为json类型的数组：

[
	{
	    "username": "张三",
	    "age": "18",
	    "sex": "男"
	  },
	{
	    "username": "张三",
	    "age": "25",
	    "sex": "男"
	  },
	{
	    "username": "马冬梅",
	    "age": "20",
	    "sex": "女"
	  }
]

第二条data内容为json类型的对象：

{
    "course":[
        {
            "id":"1",
            "course":"语文",
            "score":"75"
        },
        {
            "id":"2",
            "course":"数学",
            "score":"100"
        }
    ],
    "student":{
        "name":"小王",
        "age":"22"
    }
}

1、查询数组的长度：

// 结果为4
select jsonb_array_length(t."data"::jsonb) from test_pgsql t WHERE t.id = 1;

2、查询数组中第二个元素

//数组索引从0开始，结果为{"age": "25", "sex": "男", "username": "张三"}
select t."data"::jsonb->>1 from test_pgsql t WHERE t.id = 1;

3、查询data中所有的username

select json_array_elements(t."data"::json) #> '{username}' as guid from test_pgsql t where t."id" = 1;

//username去重
select distinct guid|| '' from (select json_array_elements(t."data"::json) #> '{username}' as guid from test_pgsql t where t."id" = 1) tmp;

结果：

4、json_object_keys 用法
说明：json_object_keys 不能用于纯数组

//获取json中的键
select json_object_keys(t."data"::json) from test_pgsql t where id = 2;

5、json_array_elements 用法

//json_array_elements用于提取转换纯数组元素，将数组拆分为单独记录
select json_array_elements(t."data"::json) from test_pgsql t WHERE t.id = 1;

6、json_extract_path 用法
说明：json_extract_path不能直接操作纯数组

//查询json中指定键(student)的值，结果为 {"name": "小王", "age": "22"}
select json_extract_path(t."data"::json, 'student') from test_pgsql t where id = 2;
//结果为 "小王"
select json_extract_path(t."data"::json, 'student','name') from test_pgsql t where id = 2;

//和 #>操作符 是一样的
select t."data"::json #>'{student}' from test_pgsql t where id = 2;
select t."data"::json #>'{student,name}' from test_pgsql t where id = 2;

示例：

新建表如下：

CREATE TABLE "public"."biz_orders" (  "ID" int8 NOT NULL DEFAULT nextval('"biz_orders_ID_seq"'::regclass),
  "info" json NOT NULL
);

表初始化语句：

INSERT INTO "biz_orders"("ID", "info") VALUES (1, '{"name":"张三","items":{"product":"啤酒","qty":6}}');
INSERT INTO "biz_orders"("ID", "info") VALUES (2, '{"name":"李四","items":{"product":"辣条","qty":8}}');
INSERT INTO "biz_orders"("ID", "info") VALUES (3, '{"name":"王五","items":{"product":"苹果","qty":18}}');
INSERT INTO "biz_orders"("ID", "info") VALUES (4, '{"name":"赵一","items":{"product":"香蕉","qty":20}}');

使用
1、简单查询

select * from biz_orders;

2、查询使用->操作符，查询json中所有顾客作为键

SELECT info -> 'name' AS customer FROM biz_orders;

3、下面使用->>操作获取所有顾客姓名作为值

SELECT info ->> 'name' AS customer FROM biz_orders;

4、根据json对象的key查询值

SELECT
   info -> 'items' ->> 'product' as product
FROM
   biz_orders
ORDER BY
   product;

5、where查询中使用json字段

SELECT
   info ->> 'name' AS customer
FROM
   biz_orders
WHERE
   info -> 'items' ->> 'product' = '辣条'

6、case 查询​​​​​​​

SELECT
   info ->> 'name' AS customer,
   info -> 'items' ->> 'product' AS product
FROM
   biz_orders
WHERE
   CAST (
      info -> 'items' ->> 'qty' AS INTEGER
   ) = 6

7、聚合函数​​​​​​​

SELECT
  MIN( CAST ( info -> 'items' ->> 'qty' AS INTEGER ) ),
  MAX( CAST ( info -> 'items' ->> 'qty' AS INTEGER ) ),
  SUM( CAST ( info -> 'items' ->> 'qty' AS INTEGER ) ),
  AVG( CAST ( info -> 'items' ->> 'qty' AS INTEGER ) ) 
FROM
  biz_orders;

8、类型查询​​​​​​​

SELECT
  json_typeof ( info -> 'items' -> 'qty' ) 
FROM
  biz_orders;