一直以来都被什么内连接、左连接、左外连接、右连接、右外连接、全连接什么的搞的糊里糊涂的,自己动手建两张表,写个sql语句查查,发现其实左连接和左外连接是一回事,右连接和右外连接也是一回事。曾经有个家伙在我面前卖弄什么左连接是这样的,左外连接又
一直以来都被什么内连接、左连接、左外连接、右连接、右外连接、全连接什么的搞的糊里糊涂的,自己动手建两张表,写个sql语句查查,发现其实左连接和左外连接是一回事,右连接和右外连接也是一回事。曾经有个家伙在我面前卖弄什么左连接是这样的,左外连接又是那样的,说的神神秘秘的,听的我糊里糊涂的。自己动手测一下,发现也就那样,用结果说明,更有说服力。
注:没有左内连接、右内连接(测试了下,报错,说明不存在这些)
创建一张student表
create table STUDENT ( ID NUMBER(5), NAME VARCHAR2(20), CLAZZID NUMBER(5) )
插入一些记录
create table CLAZZ ( ID NUMBER(5), NAME VARCHAR2(50) )插入一些记录
要求查询学生姓名和班级名称
--常用方式,不指明连接方式,默认内连接 select s.id sid,s.name 姓名,c.id cid,c.name 班名 from student s,clazz c where s.clazzid=c.id ; --等价 select s.id sid,s.name 姓名,c.id cid,c.name 班名 from student s inner join clazz c on s.clazzid=c.id ;

--左(外)连接,以左表为基表,结果集记录数等于或大于基表记录数 select s.id sid,s.name 姓名,c.id cid,c.name 班名 from student s left outer join clazz c on s.clazzid=c.id;--或者用left outer join,结果都一样 --oracle中可用+ select s.id sid,s.name 姓名,c.id cid,c.name 班名 from student s,clazz c where s.clazzid=c.id(+);--+号位于clazz,说明以student为基表

--右(外)连接,以右表为基表,结果集记录数等于或大于基表记录数 select s.id sid,s.name 姓名,c.id cid,c.name 班名 from student s right join clazz c on s.clazzid=c.id;----或者用right outer join,结果都一样 --oracle中可用+ select s.id sid,s.name 姓名,c.id cid,c.name 班名 from student s,clazz c where s.clazzid(+)=c.id;--+号位于student,说明以clazz为基表


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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