正文

009：json

米豆小书童  2022-02-14  455

关键词：

一. MySQL JSON类型

1. JSON介绍

什么是 JSON ？
- JSON 指的是 JavaScript 对象表示法（JavaScript Object Notation）
- JSON 是轻量级的文本数据交换格式
- JSON 独立于语言 *
- JSON 具有自我描述性，更易理解
MySQL5.7.8开始支持JSON数据类型。
对比存储在字符串，JSON格式的JSON数据类型提供了这些优点：
- 自动验证存储在JSON列中的JSON文档。无效的文档会产生错误。
- 优化的存储格式

官方文档(JSON类型)

2. JSON格式示例

--这个 employee 对象是包含 3 个员工记录（对象）的数组。
{
"employees": [
{ "firstName":"John" , "lastName":"Doe" }, 
{ "firstName":"Anna" , "lastName":"Smith" }, 
{ "firstName":"Peter" , "lastName":"Jones" }
]
}

3. JSON VS BLOB

JSON
- JSON数据可以做有效性检查;
- JSON使得查询性能提升;
- JSON支持部分属性索引，通过虚拟列的功能可以对JSON中的部分数据进行索引;
BLOB
- BLOB类型无法在数据库层做约束性检查;
- BLOB进行查询，需要遍历所有字符串;
- BLOB做只能做指定长度的索引;

5.7之前，只能把JSON当作BLOB进行存储。数据库层面无法对JSON数据做一些操作，只能由应用程序处理。

4.结构化和非结构化

结构化
- 二维表结构（行和列）
- 使用SQL语句进行操作
非结构化
- 使用Key-Value格式定义数据，无结构定义
- Value可以嵌套Key-Value格式的数据
- 使用JSON进行实现

--
-- SQL创建User表
--
create table user (
    id bigint not null auto_increment,
    user_name varchar(10),
    age int,
    primary key(id)
);

#
# JSON定义的User表
#

db.user.insert({
    user_name:"tom",
    age:30
})

db.createCollection("user")

5. JSON操作示例

5.1 JSON入门

--
-- 创建带json字段的表
--
CREATE TABLE t_user (
    uid BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(32) NOT NULL,
    email VARCHAR(128) NOT NULL,
    address VARCHAR(256) NOT NULL,
    UNIQUE KEY (name),
    UNIQUE KEY (email) 
)charset=utf8mb4;

mysql> INSERT INTO t_user VALUES (NULL,‘David‘,‘[email protected]‘,‘Shanghai ...‘);
INSERT INTO t_user VALUES (NULL,‘Amy‘,‘[email protected]‘,‘Beijing ...‘);
INSERT INTO t_user VALUES (NULL,‘Tom‘,‘[email protected]‘,‘Guangzhou ...‘);
Query OK, 1 row affected
Query OK, 1 row affected
Query OK, 1 row affected


mysql> select * from t_user;
+-----+-------+-------------+---------------+
| uid | name  | email       | address       |
+-----+-------+-------------+---------------+
|   1 | David | [email protected] | Shanghai ...  |
|   2 | Amy   | [email protected]   | Beijing ...   |
|   3 | Tom   | [email protected]   | Guangzhou ... |
+-----+-------+-------------+---------------+
3 rows in set

mysql> DROP TABLE IF EXISTS t_user_json;
    CREATE TABLE t_user_json(
    uid BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    data JSON
);


-- JSON_OBJECT将对象列表转化成JSON对象(key唯一,必须是偶数 key - value key - value)

mysql> insert into t_user_json 
SELECT uid,JSON_OBJECT(‘name‘,name,‘email‘,email,‘address‘,address) AS data
FROM t_user;
Query OK, 3 rows affected
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from t_user_json;
+-----+----------------------------------------------------------------------+
| uid | data                                                                 |
+-----+----------------------------------------------------------------------+
|   1 | {"name": "David", "email": "[email protected]", "address": "Shanghai ..."} |
|   2 | {"name": "Amy", "email": "[email protected]", "address": "Beijing ..."}      |
|   3 | {"name": "Tom", "email": "[email protected]", "address": "Guangzhou ..."}    |
+-----+----------------------------------------------------------------------+
3 rows in set

--JSON_EXTRACT() 是JSON提取函数，$.address 就是一个 JSON path，表示定位文档的 address 字段
JSON path 


mysql> SELECT uid,JSON_EXTRACT(data,‘$.address‘) from t_user_json;
+-----+--------------------------------+
| uid | JSON_EXTRACT(data,‘$.address‘) |
+-----+--------------------------------+
|   1 | "Shanghai ..."                 |
|   2 | "Beijing ..."                  |
|   3 | "Guangzhou ..."                |
+-----+--------------------------------+
3 rows in set


mysql> SELECT uid,JSON_EXTRACT(data,‘$.address2‘) from t_user_json;
+-----+---------------------------------+
| uid | JSON_EXTRACT(data,‘$.address2‘) |
+-----+---------------------------------+
|   1 | NULL                            |
|   2 | NULL                            |
|   3 | NULL                            |
+-----+---------------------------------+
3 rows in set

--JSON_INSERT() 是JSON 插入函数

mysql>  UPDATE t_user_json set data = json_insert(data,"$.address2","HangZhou ...") where uid = 1;
Query OK, 1 row affected
Rows matched: 1  Changed: 1  Warnings: 0
mysql> SELECT uid,JSON_EXTRACT(data,‘$.address1‘) from t_user_json;
+-----+---------------------------------+
| uid | JSON_EXTRACT(data,‘$.address1‘) |
+-----+---------------------------------+
|   1 | NULL                            |
|   2 | NULL                            |
|   3 | NULL                            |
+-----+---------------------------------+
3 rows in set


mysql> SELECT * from t_user_json;
+-----+--------------------------------------------------------------------------------------------------+
| uid | data                                                                                             |
+-----+--------------------------------------------------------------------------------------------------+
|   1 | {"name": "David", "email": "[email protected]", "address": "Shanghai ...", "address2": "HangZhou ..."} |
|   2 | {"name": "Amy", "email": "[email protected]", "address": "Beijing ..."}                                  |
|   3 | {"name": "Tom", "email": "[email protected]", "address": "Guangzhou ..."}                                |
+-----+--------------------------------------------------------------------------------------------------+
3 rows in set


-- JSON_MERGE将两个或以上的JSON对象融合 

mysql> select json_merge(JSON_EXTRACT(data,‘$.address‘) ,JSON_EXTRACT(data,‘$.address2‘)) 
from t_user_json;
+-----------------------------------------------------------------------------+
| json_merge(JSON_EXTRACT(data,‘$.address‘) ,JSON_EXTRACT(data,‘$.address2‘)) |
+-----------------------------------------------------------------------------+
| ["Shanghai ...", "HangZhou ..."]                                            |
| NULL                                                                        |
| NULL                                                                        |
+-----------------------------------------------------------------------------+
3 rows in set



mysql> begin;
UPDATE t_user_json set data = json_array_append(data,"$.address",JSON_EXTRACT(data,‘$.address2‘))
where JSON_EXTRACT(data,‘$.address2‘) IS NOT NULL AND uid >0;
select JSON_EXTRACT(data,‘$.address‘) from t_user_json;
UPDATE t_user_json set data = JSON_REMOVE(data,‘$.address2‘) where uid>0;
commit;
Query OK, 0 rows affected

Query OK, 1 row affected
Rows matched: 1  Changed: 1  Warnings: 0
+----------------------------------+
| JSON_EXTRACT(data,‘$.address‘)   |
+----------------------------------+
| ["Shanghai ...", "HangZhou ..."] |
| "Beijing ..."                    |
| "Guangzhou ..."                  |
+----------------------------------+
3 rows in set

Query OK, 1 row affected
Rows matched: 3  Changed: 1  Warnings: 0
Query OK, 0 rows affected

5.2 JSON常用函数介绍

create table demo(id int unsigned primary key auto_increment,comment json);
insert into demo(id,name) values(1,‘{"programmers": [{"email": "aaaa", "lastName": "McLaughlin", "firstName": "Brett"}, {"email": "bbbb", "lastName": "Hunter", "firstName": "Jason"}]}‘);

-- 检查第一层的key值 json_keys
-- select json_keys(comment) from demo


-- 从JSON中提取 json_extract
-- select json_extract(comment,‘$.programmers[0].email‘) from demo 


-- 从Json中去除元素 json_remove
-- select json_extract(comment,‘$.programmers‘),json_remove(comment,‘$.programmers[0]‘) from demo


-- Json是否包含当前路径 json_contains_path
-- select json_contains_path(comment,"all","$.programmers[0].firstName") from demo 


-- 判断JSON当前路径对象类型 JSON_TYPE
-- select JSON_TYPE(comment),JSON_TYPE(comment->"$.programmers[0].firstName"),JSON_TYPE(comment->"$.programmers")="ARRAY" from demo


-- 创建数组对象 JSON_ARRAY
-- SELECT JSON_ARRAY(‘a‘, 1, RAND());


-- 将对象列表转化成JSON对象(key唯一,必须是偶数 key - value key - value) JSON_OBJECT
-- SELECT JSON_OBJECT(‘key1‘, 1, ‘key2‘, ‘abc‘),JSON_OBJECT(‘key1‘, 1, ‘key2‘, ‘abc‘, ‘key1‘, ‘def‘); 


-- 将两个或以上的JSON对象融合 JSON_MERGE
-- SELECT JSON_MERGE(‘["a", 1]‘, ‘{"key": "value"}‘),JSON_MERGE(‘{"a": 2,"b":"2"}‘,‘{"key": "value"}‘),JSON_MERGE(‘{"a": 1, "b": 2}‘, ‘{"c": 3, "a": 4}‘);


-- 用户定义的变量不能JSON数据类型 函数 COLLATION(返回字符串参数的排序方式)
-- SET @j = JSON_OBJECT(‘key‘, ‘value‘); SELECT CHARSET(@j), COLLATION(@j);


-- 因为utf8mb4_bin是二进制排序规则，JSON值比较是区分大小写的。
-- SELECT JSON_ARRAY(‘x‘) = JSON_ARRAY(‘X‘);


-- JSON对大小写敏感,SQL非空不敏感
-- SELECT JSON_VALID(‘null‘), JSON_VALID(‘Null‘), JSON_VALID(‘NULL‘),ISNULL(null), ISNULL(Null), ISNULL(NULL);


-- 将JSON中元素替换;如果位置不存在，则追加 JSON_SET
-- SET @j = ‘["a", {"b": [true, false]}, [10, 20]]‘;SELECT JSON_SET(@j, ‘$[1].b[0]‘, 1, ‘$[2][3]‘, 3);


-- 向JSON中添加元素,原来位置存在数据不会替换 JSON_INSERT
-- SELECT JSON_INSERT(@j, ‘$[1].b[0]‘, 1, ‘$[2][2]‘, 2);


-- 替换JSON中原有值,不存在的不会替换 JSON_REPLACE
-- SELECT JSON_REPLACE(@j, ‘$[1].b[0]‘, JSON_OBJECT(‘key‘, ‘value‘), ‘$[2][2]‘, 2);


-- 移除JSON元素列表 JSON_REMOVE
-- SELECT JSON_REMOVE(@j, ‘$[2]‘, ‘$[1].b[1]‘, ‘$[1].b[1]‘);


-- 判断JSON中是否包含‘值‘,在这个路径下 JSON_CONTAINS
-- SELECT JSON_CONTAINS(‘{"a": 1, "b": 2, "c": {"d": 4}}‘, ‘1‘, ‘$.a‘);


-- 去掉JSON格式""号 json_unquote , ->> 这两个是等价的
-- select json_unquote(comment->"$.programmers[1].email"), comment->>"$.programmers[1].email"from demo

--
-- json_remove 从json记录中删除数据
-- 原型 : JSON_REMOVE(json_doc, path[, path] ...)
--
mysql> set @j = ‘["a", ["b", "c"], "d"]‘;   
Query OK, 0 rows affected (0.00 sec)

mysql> select json_remove(@j, ‘$[1]‘);
+-------------------------+
| json_remove(@j, ‘$[1]‘) |
+-------------------------+
| ["a", "d"]              |  -- 删除了下标为1的元素["b", "c"]
+-------------------------+
1 row in set (0.00 sec)

mysql> update user set data = json_remove(data, "$.address_2") where uid = 1;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from user;
+-----+------------------------------------------------------+
| uid | data                                                 |
+-----+------------------------------------------------------+
|   1 | {"age": 18, "name": "tom", "address": ["SZ", "BJ"]}  |  -- address_2 的字段删除了
|   2 | {"age": 28, "mail": "[email protected]", "name": "jim"}    |
|   4 | {"age": 33, "name": "jery", "email": "[email protected]"} |
+-----+------------------------------------------------------+
3 rows in set (0.00 sec)

官方文档(JSON函数)

5.3 JSON创建索引

JSON类型数据本身无法直接创建索引，需要将需要索引的JSON数据重新生成虚拟列(Virtual Columns)之后，对该列进行索引

官方文档--JSON创建索引

新建表时创建JSON索引

mysql> create table test_inex_1(
    -> data json,
    -> gen_col varchar(10) generated always as (json_extract(data, ‘$.name‘)),  -- 抽取data中的name， 生成新的一列，名字为gen_col
    -> index idx (gen_col)  -- 将gen_col 作为索引
    -> );
Query OK, 0 rows affected (0.13 sec)

mysql> show create table test_index_1；
-- -----省略表格线-----
| test_index_1 | CREATE TABLE `test_index_1` (
  `data` json DEFAULT NULL,
  `gen_col` varchar(10) GENERATED ALWAYS AS (json_extract(data, ‘$.name‘)) VIRTUAL,
  KEY `idx` (`gen_col`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
-- -----省略表格线-----
1 row in set (0.00 sec)

mysql> insert into test_index_1(data) values (‘{"name":"tom", "age":18, "address":"SH"}‘);
Query OK, 1 row affected (0.04 sec)

mysql> insert into test_index_1(data) values (‘{"name":"jim", "age":28, "address":"SZ"}‘);      
Query OK, 1 row affected (0.03 sec)

mysql> select * from test_index_1;
+---------------------------------------------+---------+
| data                                        | gen_col |
+---------------------------------------------+---------+
| {"age": 18, "name": "tom", "address": "SH"} | "tom"   |
| {"age": 28, "name": "jim", "address": "SZ"} | "jim"   |
+---------------------------------------------+---------+
2 rows in set (0.00 sec)

mysql> select json_extract(data,"$.name") as username from test_index_1 where gen_col="tom";  -- 如果这样做，为空，原因如下
Empty set (0.00 sec)

mysql> select hex(‘"‘);
+----------+
| hex(‘"‘) |
+----------+
| 22       |  -- 双引号的 16进制
+----------+
1 row in set (0.00 sec)

mysql> select hex(gen_col) from test_index_1;
+--------------+
| hex(gen_col) |
+--------------+
| 226A696D22   |  -- 双引号本身也作为了存储内容
| 22746F6D22   |
+--------------+
2 rows in set (0.00 sec)

mysql> select json_extract(data,"$.name") as username from test_index_1 where gen_col=‘"tom"‘;  -- 使用‘"tome"‘,用单引号括起来
+----------+
| username |
+----------+
| "tom"    |  -- 找到了对应的数据
+----------+
1 row in set (0.00 sec)

mysql> explain select json_extract(data,"$.name") as username from test_index_1 where gen_col=‘"tom"‘G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: test_index_1
   partitions: NULL
         type: ref 
possible_keys: idx    -- 使用了 key idx
          key: idx
      key_len: 43
          ref: const
         rows: 1
     filtered: 100.00
        Extra: NULL
1 row in set, 1 warning (0.00 sec)

---
--- 建立表的时候去掉双引用
---

mysql> create table test_index_2 (
    -> data json,
    -> gen_col varchar(10) generated always as (
    ->    json_unquote(    -- 使用json_unquote函数进行去掉双引号
    ->             json_extract(data, "$.name")
    ->    )),
    -> key idx(gen_col)
    -> );
Query OK, 0 rows affected (0.13 sec)

mysql> show create table test_index_2;
-- -----省略表格线-----
| test_index_2 | CREATE TABLE `test_index_2` (
  `data` json DEFAULT NULL,
  `gen_col` varchar(10) GENERATED ALWAYS AS (json_unquote(
            json_extract(data, "$.name")
   )) VIRTUAL,
  KEY `idx` (`gen_col`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
-- -----省略表格线-----
1 row in set (0.00 sec)

mysql> insert into test_index_2(data) values (‘{"name":"tom", "age":18, "address":"SH"}‘);
Query OK, 1 row affected (0.03 sec)

mysql> insert into test_index_2(data) values (‘{"name":"jim", "age":28, "address":"SZ"}‘);
Query OK, 1 row affected (0.02 sec)

mysql> select json_extract(data,"$.name") as username from test_index_2 where gen_col="tom";  -- 未加单引号
+----------+
| username |
+----------+
| "tom"    |  -- 可以找到数据
+----------+
1 row in set (0.00 sec)

mysql> explain select json_extract(data,"$.name") as username from test_index_2 where gen_col="tom"G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: test_index_2
   partitions: NULL
         type: ref
possible_keys: idx   -- 使用了 key idx
          key: idx
      key_len: 43
          ref: const
         rows: 1
     filtered: 100.00
        Extra: NULL
1 row in set, 1 warning (0.00 sec)

修改已存在的表创建JSON索引

--
-- 使用之前的user表操作
--
mysql> show create table user;
-- -----省略表格线-----
| user  | CREATE TABLE `user` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  `data` json DEFAULT NULL,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8mb4 |
-- -----省略表格线-----
1 row in set (0.00 sec)

mysql> select * from user;
+-----+------------------------------------------------------+
| uid | data                                                 |
+-----+------------------------------------------------------+
|   1 | {"age": 18, "name": "tom", "address": ["SZ", "BJ"]}  |
|   2 | {"age": 28, "mail": "[email protected]", "name": "jim"}    |
|   4 | {"age": 33, "name": "jery", "email": "[email protected]"} |
+-----+------------------------------------------------------+

mysql> alter table user 
    -> add user_name varchar(32)
    -> generated always as (json_extract(data,"$.name")) virtual;
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0
-- virtual 关键字是不将该列的字段值存储，对应的是stored

mysql> select user_name from user;  
+-----------+
| user_name |
+-----------+
| "tom"     |
| "jim"     |
| "jery"    |
+-----------+
3 rows in set (0.00 sec)

mysql> alter table user add index idx(user_name);          
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> select * from user where user_name=‘"tom"‘;  -- 加单引号
+-----+-----------------------------------------------------+-----------+
| uid | data                                                | user_name |
+-----+-----------------------------------------------------+-----------+
|   1 | {"age": 18, "name": "tom", "address": ["SZ", "BJ"]} | "tom"     |
+-----+-----------------------------------------------------+-----------+
1 row in set (0.00 sec)

mysql> explain select * from user where user_name=‘"tom"‘G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: user
   partitions: NULL
         type: ref
possible_keys: idx   -- 使用了 key idx
          key: idx
      key_len: 131
          ref: const
         rows: 1
     filtered: 100.00
        Extra: NULL
1 row in set, 1 warning (0.00 sec)

mysql> show create table user;
-- -----省略表格线-----
| user  | CREATE TABLE `user` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  `data` json DEFAULT NULL,
  `user_name` varchar(32) GENERATED ALWAYS AS (json_extract(data,"$.name")) VIRTUAL,
  `user_name2` varchar(32) GENERATED ALWAYS AS (json_extract(data,"$.name")) VIRTUAL,
  PRIMARY KEY (`uid`),
  KEY `idx` (`user_name`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8mb4 |
-- -----省略表格线-----
1 row in set (0.00 sec)

二. 附录

--
-- 姜boss演示JSON的SQL
--
drop table if exists User;

CREATE TABLE User (
    uid BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(32) NOT NULL,
    email VARCHAR(256) NOT NULL,
    address VARCHAR(512) NOT NULL,
    UNIQUE KEY (name),
    UNIQUE KEY (email)
);

INSERT INTO User VALUES (NULL,‘David‘,‘[email protected]‘,‘Shanghai ...‘);
INSERT INTO User VALUES (NULL,‘Amy‘,‘[email protected]‘,‘Beijing ...‘);
INSERT INTO User VALUES (NULL,‘Tom‘,‘[email protected]‘,‘Guangzhou ...‘);

SELECT * FROM User;

ALTER TABLE User ADD COLUMN address2 VARCHAR(512) NOT NULL;
ALTER TABLE User ADD COLUMN passport VARCHAR(64) NOT NULL;

DROP TABLE IF EXISTS UserJson;

CREATE TABLE UserJson(
    uid BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    data JSON
);

truncate table UserJson;

insert into UserJson 
SELECT 
    uid,JSON_OBJECT(‘name‘,name,‘email‘,email,‘address‘,address) AS data
FROM
    User;
    
SELECT * FROM UserJson; 

SELECT uid,JSON_EXTRACT(data,‘$.address2‘) from UserJson;
    
UPDATE UserJson
set data = json_insert(data,"$.address2","HangZhou ...")
where uid = 1;

SELECT JSON_EXTRACT(data,‘$.address[1]‘) from UserJson;

select json_merge(JSON_EXTRACT(data,‘$.address‘) ,JSON_EXTRACT(data,‘$.address2‘)) 
from UserJson;

begin;
UPDATE UserJson
set data = json_array_append(data,"$.address",JSON_EXTRACT(data,‘$.address2‘))
where JSON_EXTRACT(data,‘$.address2‘) IS NOT NULL AND uid >0;
select JSON_EXTRACT(data,‘$.address‘) from UserJson;
UPDATE UserJson
set data = JSON_REMOVE(data,‘$.address2‘)
where uid>0;
commit;

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set集合set集合：作用：集合之间的交差并补；特点：无序，自动去重；对于set的值是可哈希的数值，例如：intfloatboolcomplexstrtuple定义集合setvar=set()#空集合setvar=‘北京‘,‘武汉‘,‘内蒙古‘print(setvar,type(setvar))?结果：‘内蒙古... 查看详情

009-快速构建一个业务模块(代码片段)

视频讲解地址：009-快速构建一个业务模块(3)_哔哩哔哩_bilibili009-快速构建一个业务模块(3)https://www.bilibili.com/video/BV1w34y1i7dp?spm_id_from=333.999.0.0本演示讲解一个模拟业务模块从导入到配置的过程，使您可以快速了解本系统... 查看详情

009-快速构建一个业务模块(代码片段)

atcodergrandcontest009题解

A-MultipleArray倒着算要加多少就好了。//waz#include<bits/stdc++.h>usingnamespacestd;#definempmake_pair#definepbpush_back#definefifirst#definesesecond#defineALL(x)(x).begin(),(x).end()#defineSZ(x)((int)((x). 查看详情

009错误处理(代码片段)

一.概述　　在springboot之中,为我们的默认的错误处理进行了默认的配置,本此我们来看看错误处理的方式. 二.错误处理的原理　　我们首先找到错误的动配置类之中.@Configuration@ConditionalOnWebApplication@ConditionalOnClass(Servlet.class,Disp... 查看详情

练习009：转换小写字母

题目：解答：小机灵鬼23333注意：ECMAScript中涉及字符串大小写转换的方法有4个：toLowerCase()、toLocaleLowerCase()、toUpperCase()和toLocaleUpperCase()。其中，toLowerCase()和toUpperCase()是两个经典的方法，借鉴自java.lang.String中的同名方法。而toLo... 查看详情

lesson009lesson010作业

lesson009 作业厘米to 英尺代码：1#include<stdio.h>2intmain()3{4//1100厘米到英尺5//2printf11*0.03286//3printf22*0.03287//8for(inti=1;i<=100;i++)9{10printf("%d厘米=%f英尺 ",i,i*0.0328);11}12getchar();1 查看详情

009.分布式事务

数据库事务：　　ACID分布式理论：　　CAP定理　　BASE理论分布式事务解决方案：　　两阶段提交（2PC）　　补偿事务（TCC）　　本地消息表（异步确认）　　MQ事务消息　　https://www.cnblogs.com/savorboard/p/distributed-system-transaction-con... 查看详情

rubyrubysteps009-oop-eastyoop（摘录）(代码片段)

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