内连接 1 语法 SELECT 列名 FROM 表1 INNER JOIN 表2 ON 表1.列名 条件运算符 表2.列名 [WHERE 条件] [ORDER BY 排序列] 其中ON 表1.列名 条件运算符 表2.列名 中,条件运算符常用的是:=、。 表1.列名和表2.列名,分别是两个表的公共列。 内连接查询出的是两
内连接
1 语法
SELECT
FROM 表1
INNER JOIN 表2 ON 表1.列名 条件运算符 表2.列名
[WHERE 条件]
[ORDER BY 排序列]
其中“ON 表1.列名 条件运算符 表2.列名” 中,条件运算符常用的是:=、。
表1.列名和表2.列名,分别是两个表的公共列。
内连接查询出的是两个表公共列共有的记录。
2 示例
(1)Books与Authors内连接SQL语句
<span>SELECT</span> b.BookCode <span>as</span> 图书编号, b.BookName <span>as</span> 图书名称, a.AuthorName <span>as</span><span> 作者姓名 </span><span>FROM</span> Books <span>as</span><span> b </span><span>INNER</span> <span>JOIN</span> Authors <span>as</span> a <span>ON</span> b.AuthorID <span>=</span> a.AuthorID
(2)内连接另外一种写法
<span>SQL Server2005还兼容以前版本的内连接语法,即使用WHERE子句进行内连接。 </span><span>SELECT</span> b.BookCode <span>as</span> 图书编号, b.BookName <span>as</span> 图书名称, a.AuthorName <span>as</span><span> 作者姓名 </span><span>FROM</span><span> Books b, Authors a </span><span>WHERE</span> b.AuthorID <span>=</span> a.AuthorID
3 多表内连接语法
SELECT
FROM 表1
INNER JOIN 表2 ON 表1.列名 条件运算符 表2.列名
INNER JOIN 表3 ON 表1.列名 条件运算符 表3.列名
……
[WHERE 条件]
[ORDER BY 排序列]
4 示例
<span>SELECT</span> b.BookCode <span>as</span> 图书编号, c.PublisherName <span>as</span><span> 出版商名, b.BookName </span><span>as</span> 图书名称,a.AuthorName <span>as</span><span> 作者姓名 </span><span>FROM</span> Books b <span>INNER</span> <span>JOIN</span> Authors a <span>ON</span> b.AuthorID<span>=</span><span>a.AuthorID </span><span>INNER</span> <span>JOIN</span> Publisher c <span>ON</span> b.PublisherID<span>=</span>c.PublisherID
外连接
左外连接语法
SELECT
FROM 左表
LEFT [OUTER] JOIN 右表 ON 左表.列名 条件运算符 右表.列名
[WHERE 条件]
[ORDER BY 排序列]
示例
使用左外连接查询Books与Authors表中的数据
<span>SELECT</span> b.BookCode <span>as</span> 图书编号, b.BookName <span>as</span> 图书名称, a.AuthorName <span>as</span><span> 作者姓名 </span><span>FROM</span><span> Books b </span><span>LEFT</span> <span>JOIN</span> Authors a <span>ON</span> b.AuthorID<span>=</span>a.AuthorID
左外连接是以左表为主表,去关联右表(从表),结果集中包含主表所有数据行,如果主表的某行在从表中没有匹配行时,则从表的选择列为NULL值。
右外连接语法
SELECT
FROM 左表 RIGHT [OUTER] JOIN 右表
ON 左表.列名 条件运算符 右表.列名
[WHERE 条件]
[ORDER BY 排序列]
示例
使用右外连接查询Books与Authors表中的数据
SELECT b.BookCode as 图书编号, b.BookName as 图书名称, a.AuthorName as 作者姓名
FROM Books b
RIGHT JOIN Authors a ON b.AuthorID=a.AuthorID
右外连接是以右表为主表,去关联左表(从表),结果集中包含主表所有数据行,如果主表的某行在从表中没有匹配行时,则从表的选择列为NULL值。
完全连接
语法
SELECT
FROM 左表
FULL [OUTER] JOIN 右表 ON 左表.列名 条件运算符 右表.列名
[WHERE 条件]
[ORDER BY 排序列]
完全连接左表和右表中所有行,当某行数据在另一个表中没有匹配时,则另一个表的选择列值为NULL
示例
使用完全连接查询Books与Authors表中的数据
<span>SELECT</span> b.BookCode <span>as</span> 图书编号, b.BookName <span>as</span> 图书名称, a.AuthorName <span>as</span><span> 作者姓名 </span><span>FROM</span><span> Books b </span><span>FULL</span> <span>JOIN</span> Authors a <span>ON</span> b.AuthorID<span>=</span>a.AuthorID
交叉联接
概念:没有用where子句的交叉连接将产生连接所涉及的笛卡尔积第一个表的行数乘以第二个表的行数等于笛卡尔积和结果集的大小
交叉连接: Cross join(不带条件where,如果带返回或显示的是匹配的行数)
SQL语法:select * from Books cross join Books
如果有条件(<span>where</span><span>) </span><span>select</span> <span>*</span> <span>from</span> Books <span>cross</span> <span>join</span> Authors <span>where</span> Books.AuthorID<span>=</span><span> Authors.AuthorID 等价于 </span><span>select</span> <span>*</span> <span>from</span> Books ,Authors <span>--</span><span>(不带where)</span>

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


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Dreamweaver Mac version
Visual web development tools

WebStorm Mac version
Useful JavaScript development tools

Safe Exam Browser
Safe Exam Browser is a secure browser environment for taking online exams securely. This software turns any computer into a secure workstation. It controls access to any utility and prevents students from using unauthorized resources.

DVWA
Damn Vulnerable Web App (DVWA) is a PHP/MySQL web application that is very vulnerable. Its main goals are to be an aid for security professionals to test their skills and tools in a legal environment, to help web developers better understand the process of securing web applications, and to help teachers/students teach/learn in a classroom environment Web application security. The goal of DVWA is to practice some of the most common web vulnerabilities through a simple and straightforward interface, with varying degrees of difficulty. Please note that this software

MantisBT
Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.