3.3 创建、删除和选择数据库
MySQL提供了三条数据库级的语句,它们分别是: CREATE DATABASE 用于创建数据库,DROP DATABASE 用于删除数据库,USE 用于选择缺省数据库。
1. CREATE DATABASE 语句
创建一个数据库很容易;只要在CREATE DATABASE 语句中给出其名称即可:
其中限制条件是该数据库的名称必须是合法的,该数据库必须不存在,并且您必须有足够的权限来创建它。
2. DROP DATABASE 语句
删除数据库就像创建它一样容易,假如有权限,执行下列语句即可:
请注意,不要乱用DROP DATABASE 语句,它将会删除数据库及其所有的表。在删除了一个数据库后,该数据库就永远没有了。换句话说,不要仅为了看看这条语句如何工作就试着执行该语句。如果管理员已经正常完成了数据库备份,那么删除的数据库可能还可以恢复。
请注意,数据库是由数据目录中的一个目录表示的。如果在该目录中放置了一些非表的数据文件,它们是不会被DROP DATABASE 语句删除的。此时,该数据库目录自身也不被删除。
3. USE 语句
USE 语句选择一个数据库,使其成为服务器的给定连接的缺省(当前)数据库:
必须对数据库具有某种访问权限,否则不能使用它。为了使用数据库中的表而选择该数据库实际上不是必须的,因为可以利用db_name.tbl_name 形式来引用它的表。但是,不必指定数据库限定词引用表要方便得多。选择一个缺省数据库并不代表在连接的持续时间内它都必须是缺省的。可发布任意数目的USE 语句在数据库之间进行任意地切换,只要具有使用它们的权限即可。选择一个数据库也不限制您只使用该数据库中的表。您仍然可以通过用数据库名限定表名的方法,引用其他数据库中的表。
在服务器的连接终止时,服务器关于缺省数据库的所有记忆都消失了。即,如果您再次连接到该服务器,它不会记住以前您所选择的数据库。事实上,假定MySQL是多线程的,可通过一个用户处理多个连接,用户可以按任何顺序连接或断开,让服务器对缺省数据库进行记忆的想法也是没有意义的。在这个环境中,“以前选择的数据库”这句话指什么并不清楚。

MySQL's position in databases and programming is very important. It is an open source relational database management system that is widely used in various application scenarios. 1) MySQL provides efficient data storage, organization and retrieval functions, supporting Web, mobile and enterprise-level systems. 2) It uses a client-server architecture, supports multiple storage engines and index optimization. 3) Basic usages include creating tables and inserting data, and advanced usages involve multi-table JOINs and complex queries. 4) Frequently asked questions such as SQL syntax errors and performance issues can be debugged through the EXPLAIN command and slow query log. 5) Performance optimization methods include rational use of indexes, optimized query and use of caches. Best practices include using transactions and PreparedStatemen

MySQL is suitable for small and large enterprises. 1) Small businesses can use MySQL for basic data management, such as storing customer information. 2) Large enterprises can use MySQL to process massive data and complex business logic to optimize query performance and transaction processing.

InnoDB effectively prevents phantom reading through Next-KeyLocking mechanism. 1) Next-KeyLocking combines row lock and gap lock to lock records and their gaps to prevent new records from being inserted. 2) In practical applications, by optimizing query and adjusting isolation levels, lock competition can be reduced and concurrency performance can be improved.

MySQL is not a programming language, but its query language SQL has the characteristics of a programming language: 1. SQL supports conditional judgment, loops and variable operations; 2. Through stored procedures, triggers and functions, users can perform complex logical operations in the database.

MySQL is an open source relational database management system, mainly used to store and retrieve data quickly and reliably. Its working principle includes client requests, query resolution, execution of queries and return results. Examples of usage include creating tables, inserting and querying data, and advanced features such as JOIN operations. Common errors involve SQL syntax, data types, and permissions, and optimization suggestions include the use of indexes, optimized queries, and partitioning of tables.

MySQL is an open source relational database management system suitable for data storage, management, query and security. 1. It supports a variety of operating systems and is widely used in Web applications and other fields. 2. Through the client-server architecture and different storage engines, MySQL processes data efficiently. 3. Basic usage includes creating databases and tables, inserting, querying and updating data. 4. Advanced usage involves complex queries and stored procedures. 5. Common errors can be debugged through the EXPLAIN statement. 6. Performance optimization includes the rational use of indexes and optimized query statements.

MySQL is chosen for its performance, reliability, ease of use, and community support. 1.MySQL provides efficient data storage and retrieval functions, supporting multiple data types and advanced query operations. 2. Adopt client-server architecture and multiple storage engines to support transaction and query optimization. 3. Easy to use, supports a variety of operating systems and programming languages. 4. Have strong community support and provide rich resources and solutions.

InnoDB's lock mechanisms include shared locks, exclusive locks, intention locks, record locks, gap locks and next key locks. 1. Shared lock allows transactions to read data without preventing other transactions from reading. 2. Exclusive lock prevents other transactions from reading and modifying data. 3. Intention lock optimizes lock efficiency. 4. Record lock lock index record. 5. Gap lock locks index recording gap. 6. The next key lock is a combination of record lock and gap lock to ensure data consistency.


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