• Mybatis query error identifying class… Types() or values ()


    We encountered this problem when we learned the relationship of mybatis. Error instantiating class com.springmvc_mybatis.pojo.Items with invalid types () or values (). Instantiate a class with an invalid type () or value() com.springmvc_ mybatis.pojo.items There was an error while reading terms of settlement: Instantiation failed, which indicates that there is something wrong with the constructor […]

  • Spark of big data framework


    Spark cluster composition and execution process 1、 Spark cluster compositionSpark cluster is composed of cluster manager, worker, executor, driver and application.1. Driver: this process executes the main method of Spark Program and is responsible for the execution of actual code.1) Start sparkcontext or sparksession to convert user program into job;2) Scheduling tasks between executors;3) Track […]

  • Warning not scoped to the top level: root element appears when swiper is introduced


    After introducing swiper into the project, we found that many warnings similar to the following appeared in the console warning in ./node_modules/swiper/css/swiper.css Module Warning (from ./node_modules/postcss-loader/src/index.js): Warning (429:3) Custom property ignored: not scoped to the top-level :root element (.swiper-lazy-preloader-black { … –swiper-preloader-color: … }) @ ./node_modules/swiper/css/swiper.css 4:14-135 14:3-18:5 15:22-143 @ ./node_modules/cache-loader/dist/cjs.js??ref–12-0!./node_modules/babel-loader/lib!./node_modules/cache-loader/dist/cjs.js??ref–0-0!./node_modules/vue-loader/lib??vue-loader-options!./src/views/payment/pay.vue?vue&type=script&lang=js& @ ./src/views/payment/pay.vue?vue&type=script&lang=js& @ ./src/views/payment/pay.vue […]

  • Relational algebra


    Relational algebra Abstract query language, using relational operations to express queries Operands: relationships Operation result: relation       Traditional relational operation (set relational operation) 1. Union operation (ν) R ν s: put the two relations together and eliminate the repetition   2. Intersection (∩) R ∩ s: the repetitive part of two relations   […]

  • 50 years later, why is SQL still so important?


    Translated by: Yuan Yuhan In March 1971, Intel launched the world’s first general-purpose microprocessor, [Intel 4004], which costs $60 and has about 2300 transistors. Today, the latest iPhone has nearly 12 billion transistors and costs just over $60. The changes brought about by 50 years are huge, but some things remain unchanged. In the 1990s, […]

  • MySQL – getting to know MySQL


    A series of articles: MySQL – getting to know MySQL Operation database Modify delete table Create database table Data management and DML statement of MySQL DQL (date query language) database query statement affair Indexes JDBC Database operation Database connection pool affair MySQL advanced optimization 1.1 why use database Database (store data)Txt, Excel, word can also […]

  • Warshall relation transitive closure


    definition Transitive closureThat is to say, in mathematics, the transitive closure of binary relation R on set X is the smallest transitive relation on X containing R. Related issues Calculate the transitive relation in a matrix (e.g. the connected relation between two points on a graph) Warshall algorithm is usually used for calculation The calculation […]

  • Advantages, disadvantages and concepts of Java redis Series 1 relational database and non relational database


    Advantages, disadvantages and concepts of Java redis Series 1 relational database and non relational database Before learning redis, let’s first learn two concepts: what is a relational database and what is a non relational database, what is the difference between the two, and what is the relationship between the two? ** Relational database **Relational database […]

  • Coder: a brief history of databases


    The author of this paper: coder, senior developer, keen on open source community,GitHubStar of 20K, follower of 15K; believer of digital currency, who loves secondary market trading (stock + digital currency); currently investing in Zhenge fund. If you want to communicate with the coder, you can add his wechat: daimajia (famous source and identity). If […]

  • Full analysis of blockmanager in Spark’s distributed storage system


    Abstract:Blockmanager is a very important component in spark. In the process of spark running, blockmanager is everywhere. Only by understanding the principle and mechanism of blockmanager, can you have a deeper understanding of spark. What is blockmanager? What is the role of blockmanager? I understand that it is responsible for the storage of RDD and […]

  • Introduction and installation of redis framework (MAC and windows)


    A series of articles: Installing redis under Linux Introduction and use of redis Introduction and installation of redis framework (MAC and windows) Redis installation and use Redis introduction Our daily development, data need to be persistent storage, common persistent storage has many kinds, such as database, file, computer memory, and even cloud server are persistent […]

  • Is relational database still available? Who can take over NoSQL or newsql?


    This article is from: dbaplus community Introduction to the author Zhang Liang, head of data research and development of Jingdong Digital Technology Co., Ltd. He loves open source and currently leads two open source projects elastic job and sharding sphere (sharding JDBC). He is good at distributed architecture based on Java and cloud platform based […]