Why does JavaScript have null? (Translation) - Tianbeiya

Do you want to know how to display child categories on the parent category archive page? When you customize a classification archive page, you may need to do this to make it more useful to your visitors. In this article, we will show you how to easily display child categories on the parent category archive page. Why do subcategories appear on parent category archive page? By displaying all child categories on the parent category archive page, you can make them less generic and more useful to visitors. For example, if you run a WordPress blog about books and have a taxonomy called "Theme", you can add sub-taxonomy such as "novel", "non-fiction" so that your readers can

The key to installing MySQL elegantly is to add the official MySQL repository. The specific steps are as follows: Download the MySQL official GPG key to prevent phishing attacks. Add MySQL repository file: rpm -Uvh https://dev.mysql.com/get/mysql80-community-release-el7-3.noarch.rpm Update yum repository cache: yum update installation MySQL: yum install mysql-server startup MySQL service: systemctl start mysqld set up booting

Recovering deleted rows directly from the database is usually impossible unless there is a backup or transaction rollback mechanism. Key point: Transaction rollback: Execute ROLLBACK before the transaction is committed to recover data. Backup: Regular backup of the database can be used to quickly restore data. Database snapshot: You can create a read-only copy of the database and restore the data after the data is deleted accidentally. Use DELETE statement with caution: Check the conditions carefully to avoid accidentally deleting data. Use the WHERE clause: explicitly specify the data to be deleted. Use the test environment: Test before performing a DELETE operation.

CentOS will be shut down in 2024 because its upstream distribution, RHEL 8, has been shut down. This shutdown will affect the CentOS 8 system, preventing it from continuing to receive updates. Users should plan for migration, and recommended options include CentOS Stream, AlmaLinux, and Rocky Linux to keep the system safe and stable.

The core of Oracle SQL statements is SELECT, INSERT, UPDATE and DELETE, as well as the flexible application of various clauses. It is crucial to understand the execution mechanism behind the statement, such as index optimization. Advanced usages include subqueries, connection queries, analysis functions, and PL/SQL. Common errors include syntax errors, performance issues, and data consistency issues. Performance optimization best practices involve using appropriate indexes, avoiding SELECT *, optimizing WHERE clauses, and using bound variables. Mastering Oracle SQL requires practice, including code writing, debugging, thinking and understanding the underlying mechanisms.

Oracle database migration mainly relies on expdp and impdp tools. 1. expdp is used to export data. Its syntax is concise but has rich options. Pay attention to directory permissions and file size to avoid export failures. 2. impdp is used to import data. It is necessary to ensure that the target database space is sufficient, the character set is consistent and there are no objects with the same name. The remap_schema parameter can be used to resolve conflicts. 3. Parallel, query, network_link, exclude and other parameters can be used to optimize the migration process; 4. Large database migration requires attention to network environment, database resource utilization and batch migration strategies to improve efficiency and reduce risks. Only by mastering these steps and techniques can you

The main tools for connecting to MongoDB are: 1. MongoDB Shell, suitable for quickly viewing data and performing simple operations; 2. Programming language drivers (such as PyMongo, MongoDB Java Driver, MongoDB Node.js Driver), suitable for application development, but you need to master the usage methods; 3. GUI tools (such as Robo 3T, Compass) provide a graphical interface for beginners and quick data viewing. When selecting tools, you need to consider application scenarios and technology stacks, and pay attention to connection string configuration, permission management and performance optimization, such as using connection pools and indexes.

Docker uses Linux kernel features to provide an efficient and isolated application running environment. Its working principle is as follows: 1. The mirror is used as a read-only template, which contains everything you need to run the application; 2. The Union File System (UnionFS) stacks multiple file systems, only storing the differences, saving space and speeding up; 3. The daemon manages the mirrors and containers, and the client uses them for interaction; 4. Namespaces and cgroups implement container isolation and resource limitations; 5. Multiple network modes support container interconnection. Only by understanding these core concepts can you better utilize Docker.