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JAVA 8 Spring Boot 2.5.3 MySQL 5.7.21 ---   Note: this code uses org.projectlombok:lombok (the version is specified by Spring Boot).   catalogue 1. Create entity class 2. Primary key id 3. Composite primary key 4. General field 4.1. Use @ Column annotation 4.2. Do not use @ Column annotation 4.3 enumeration type 4.4. Creation time and last upda ...

I failed to install Mysql on Linux for many times, which was very painful. So I wrote a Mysql installation tutorial at night, hoping to avoid detours in the installation! 1, Package installation        1,   First, prepare a clean Linux server, which should be emphasized here. Don't miss the Bug [root@VM-0-2-centos ...

join query MySQL connection query usually combines the record rows from two or more tables to splice data based on the common fields between these tables. First, determine a main table as the result set, and then selectively connect the rows of other tables to the selected main table result set. Connection queries that are frequently used ...

1, Database function 1. Mathematical function The records stored in the database often need a series of arithmetic operations, so MySQL supports many mathematical functions. Common mathematical functions Mathematical functiondescribeabs (x)Returns the absolute value of x; The absolute value is always positive, and the absolute value o ...

introduction With the rapid development of office automation and e-commerce, enterprises rely more and more on information system. Database, as the core of information system, plays an important role. Database backup is an important means to recover important data in time and prevent data loss in case of data loss. A reasonable database ...

1, Introduction to mysql lock Lock source When there are concurrent operations, a mechanism is necessary to ensure the integrity and consistency of data. Lock is the realization of this technology. Lock type According to the concept, it is divided into pessimistic lock and optimistic lock According to particle size: Table lock, page ...

hadoop startup error ERROR: Cannot set priority of datanode process The cluster was not formatted ./bin/hdfs namenode -format Modify yarn default port After that, it needs to be restarted because it is not hot database Error reporting 1045 [root@test ~]# systemctl stop mariadb ##Close mysql [root@test ~]# mysqld_safe --skip-grant-tables ...

MySql database is a relational database MySQL database is not case sensitive Some grammars in this article, such as goudan, dachui and Cuihua, are not fixed grammars and can be named arbitrarily data typeintegerint(m) m indicates the lengthLong integerbigint(m)Floating point numberdouble(m,d) m represents the total length d and the decimal po ...

JDBC and DBUtils tools What is JDBC? In Web development, it is inevitable to use database to store and manage data. In order to provide support for database access in Java language, SUN company provided a set of standard Java class library for database access, namely JDBC, in 1996 The full name of JDBC is Java Database Connectivity, which is ...

CRUD is what we often call addition, deletion, query and modification. It is also the most basic operation of the database C: create add R: retrieve find U: update modification D: Delete delete Comment: in SQL - indicates a comment, which needs to be on a single line, or use comment comment. 1, Create Let's create a student table first ...