默认情况下,所有列的值都可以包含null值,当在列上定义not null约束后,列上面就必须得有值。not null约束还常常与其它的约束一
1,约束的分类。
约束分成5类:1. not null,2.primary key,3.check,4.unique,5.foreign key。
1.1 not null约束
默认情况下,所有列的值都可以包含null值,当在列上定义not null约束后,,列上面就必须得有值。not null约束还常常与其它的约束一起组合起来使用,比如与unique约束一起使用,就可以保证新插入的列的数据不会与已经存在的数据发生冲突。需要在相当的列上面创建索引的时候,建议也在相关的列上面增加上not null约束,因为索引不会存放null记录。
1.2 primary key约束
主键约束其实就是not null约束与unique约束的一个组合,用来保证行记录的唯一,不重复性。每张表只能有一个主键约束,在表设计的时候,我们一般都每张表上面都要有主键约束。在创建主键的时候会自己的创建相应约束名的索引,在选择主键约束的列的时候可以参考下面指导:
1.选择sequence的列做为主键。
2.选择列的值是唯一的,并且没有null值的列。
3.主键的更一般不会发生修改,也仅仅用于标识行的唯一性,不用于其它的目的。
4.主键的列尽量选择值比较短的值或者是number的值。
1.3 unique约束
unique约束是保证值的记录不会出现相同的值,但是noll值不受权限,创建unique约束的时候,会自己创建约束名的索引。
1.4 check约束
检查约束用于检查值在插入时是否满足指定的条件,比如值要求大于10小于100.
1.5 foreign key约束
当2个表,当A表中的列的值必须在B表中的列的值时候,可以定义外键约束。父表相关的值上面有主键或者唯一性约束。不过很多公司要求不能使用外键,让开发自己用程序来判断。
2,约束的定义
约束可以在表创建的时候指定,也可以在表创建完成后通过alter命令来创建,下面是每一种约束创建的语法。

InnoDBBufferPool reduces disk I/O by caching data and indexing pages, improving database performance. Its working principle includes: 1. Data reading: Read data from BufferPool; 2. Data writing: After modifying the data, write to BufferPool and refresh it to disk regularly; 3. Cache management: Use the LRU algorithm to manage cache pages; 4. Reading mechanism: Load adjacent data pages in advance. By sizing the BufferPool and using multiple instances, database performance can be optimized.

Compared with other programming languages, MySQL is mainly used to store and manage data, while other languages such as Python, Java, and C are used for logical processing and application development. MySQL is known for its high performance, scalability and cross-platform support, suitable for data management needs, while other languages have advantages in their respective fields such as data analytics, enterprise applications, and system programming.

MySQL is worth learning because it is a powerful open source database management system suitable for data storage, management and analysis. 1) MySQL is a relational database that uses SQL to operate data and is suitable for structured data management. 2) The SQL language is the key to interacting with MySQL and supports CRUD operations. 3) The working principle of MySQL includes client/server architecture, storage engine and query optimizer. 4) Basic usage includes creating databases and tables, and advanced usage involves joining tables using JOIN. 5) Common errors include syntax errors and permission issues, and debugging skills include checking syntax and using EXPLAIN commands. 6) Performance optimization involves the use of indexes, optimization of SQL statements and regular maintenance of databases.

MySQL is suitable for beginners to learn database skills. 1. Install MySQL server and client tools. 2. Understand basic SQL queries, such as SELECT. 3. Master data operations: create tables, insert, update, and delete data. 4. Learn advanced skills: subquery and window functions. 5. Debugging and optimization: Check syntax, use indexes, avoid SELECT*, and use LIMIT.

MySQL efficiently manages structured data through table structure and SQL query, and implements inter-table relationships through foreign keys. 1. Define the data format and type when creating a table. 2. Use foreign keys to establish relationships between tables. 3. Improve performance through indexing and query optimization. 4. Regularly backup and monitor databases to ensure data security and performance optimization.

MySQL is an open source relational database management system that is widely used in Web development. Its key features include: 1. Supports multiple storage engines, such as InnoDB and MyISAM, suitable for different scenarios; 2. Provides master-slave replication functions to facilitate load balancing and data backup; 3. Improve query efficiency through query optimization and index use.

SQL is used to interact with MySQL database to realize data addition, deletion, modification, inspection and database design. 1) SQL performs data operations through SELECT, INSERT, UPDATE, DELETE statements; 2) Use CREATE, ALTER, DROP statements for database design and management; 3) Complex queries and data analysis are implemented through SQL to improve business decision-making efficiency.

The basic operations of MySQL include creating databases, tables, and using SQL to perform CRUD operations on data. 1. Create a database: CREATEDATABASEmy_first_db; 2. Create a table: CREATETABLEbooks(idINTAUTO_INCREMENTPRIMARYKEY, titleVARCHAR(100)NOTNULL, authorVARCHAR(100)NOTNULL, published_yearINT); 3. Insert data: INSERTINTObooks(title, author, published_year)VA


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