Programmer Group

Data management of MySQL 1.DML language (very important) The meaning of database: data storage and data management DML: Data Manipulation Language insertupdatedelete 2. Foreign keys Method 1: add constraints to the primary key when creating a table (more complex) CREATE TABLE `grade`( `gradeid` INT(10) NOT NULL AUTO_INCREMENT CO ...

mybatis learning notes, 2.MyBatisTest 1. Introduction 1. Interface programming Native: Dao = = = = > daoimplmybatis: Mapper ====> xxMapper.xml 2. SqlSession represents a session with the database; It must be closed after use; 3. SqlSession, like connection, is non thread safe. You should get a new object every time you use it. ...

Aggregate query Aggregate query refers to the statistical query of part or all of the data of a field in a data table. Aggregate query is required to query the average price of all books in the book data table and the total number of all books. Whole table aggregation Import method: from django.db.models import* Aggregate function: Sum, Avg, ...

0. Project construction As usual, let's build a new project first   configuration   The project is a little messy. The name of the project is what we call it   mybatis-03-mapper   Well, the imported dependencies are the same as before, and then simplify the previous code. The copied project structure and documents shall be ...

catalogue 1. Brief 2. Development environment and tools 3. Background password encryption 3.1 encryption code 3.2 SM3 encryption class (Sm3crypto) 3.3 state secret SM3 tools (Sm3Utils) 3.4 dependence package related to state secrets 4. Login authentication 4.1 front end key codes 4.2 key codes of backend login authentication 4.2.1.SM ...

1, Basic command 1. Switch database (select) redis has 16 databases by default, and the 0 database is used by default Command for switching data: select number localhost:6379> select 2 OK # Switching succeeded localhost:6379[2]> # 6379 [2] represents the third database 2. set and get methods Command 1: set ...

Introduction to character sets and comparison rules Character set introduction We know that only binary data can be stored in the computer, so how to store strings? Of course, it is necessary to establish the mapping relationship between characters and binary data. To establish this relationship, at least two things should be clear: What ch ...

1. Select installation package The source file directory can be found in the official of postgresql. The address is as follows: Official website address of postgresql , select the corresponding version for installation according to the project requirements, as shown in the figure below: 2. Installation ① Unzip the installation package First, ...

Mybatis plus upgrade complete! What has my system been updated? What's better about mybatis plus than mybatis? I've been writing about the system for nearly a month. Generally speaking, I upgraded my Mybatis to Mybatis plus, but it's not exactly like this. There are many small changes. This time, I specially wrote a blog to record the update ...

2021SC@SDUSC 1, Brief description Coding + compression can double the disk space of data and save considerable storage costs. At the same time, shrinking can usually improve the system throughput, so that the system can do more work In terms of space saving processing at the storage level, HBase provides two schemes: 1. Key based coding. I ...