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1, One to one mapping Suppose that in the permission system, a user can only have one role, that is, the relationship between users and roles is limited to one-to-one relationship. 1.1 handling one-to-one relationships using automatic mapping A user has a role. We add the role class SysRole field in the user class SysUser. As follows: publi ...

1. Classification of sub query 1.1. Non associated subquery Subquery statements can be executed separately without reference to any content in the included statement. It is called non associated subquery. 1.1.1. Single row single column subquery The subquery statement returns a single row and single column table. This type of subque ...

JAVA program deployment preface Operating environment: Galaxy Kirin + Feiteng CPU Project framework: Springboot + Mybatis + Dameng database jdk1 eight The last one wrote the source code of the project, and this one mainly wrote the deployment of the project on the domestic operating system and the installation and creation of Dameng data ...

1, Explain tool introduction Use the EXPLAIN keyword to simulate the optimizer to execute SQL statements and analyze the performance bottlenecks of your query statements or structures Add the explain keyword before the select statement. MySQL will set a flag on the query. Executing the query will return the information of the execution plan i ...

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The original content of GreatSQL community cannot be used without authorization. Please contact Xiaobian and indicate the source for reprint.[toc]This paper introduces how to use GreatSQL 8.0.25 to build a three node MGR cluster.1. Installation PreparationPrepare the following three servers:IPportrole172.16.16.103306mgr1172.16.16.113306mgr2172. ...

   🎈 Write in front 🙋‍♂️ Hello everyone, I'm super dream. You can call me Xiaomeng~ We all know that we have to deal with the database almost every day, whether in study or daily work. In order to better operate the database, our SQL knowledge reserve is essential. If you want to master SQL well, you must practice and stu ...

Previously reviewed: SQL knowledge points -- insert records SQL knowledge points -- update records and delete records Basic skills of SQL (I) -- SQL and database Basic skills of SQL (II) -- basic query Basic skills of SQL (III) -- aggregation and sorting catalogue view Advantages of view Delete view Subquery scalar subquery ...

Single Line Processing Function Data processing functions are also known as one-line processing functions Features of one-line processing functions: one input corresponds to one output. As opposed to single-line processing functions, multiline processing functions. (Multi-line processing function features: multiple inputs, corresponding to o ...