mysql连接查询_MySQL
一:新建两张表
student表
CREATE TABLE student(
idINT PRIMARY KEY AUTO_INCREMENT,
snameVARCHAR(20) NOT NULL
);
SELECT * FROM student;
INSERT INTO student VALUES (1,'张三'),(2,'李四'),(3,'王五');
course表
CREATE TABLE course(
idINT PRIMARY KEY,
cnameVARCHAR(20) NOT NULL
);
SELECT * FROM course;
INSERT INTO course VALUES (1,'足球'),(2,'音乐'),(4,'美术');
二:外连接
外连接可分为:左连接、右连接、完全外连接。
1. 左连接 left join或者left outer join
SELECT * FROM student LEFT JOIN course ONstudent.id=course.id;
id sname id cname
1 张三 1 足球
2 李四 2 音乐
3 王五 \N \N
左外连接包含left join左表所有行,如果左表中某行在右表没有匹配,则结果中对应行右表的部分全部为空(NULL).
注:此时我们不能说结果的行数等于左表数据的行数。当然此处查询结果的行数等于左表数据的行数,因为左右两表此时为一对一关系。
2.右连接 right join 或 right outer join
SELECT * FROM student RIGHT JOIN course ONstudent.id=course.id;
id sname id cname
1 张三 1 足球
2李四 2音乐
\N \N 4美术
右外连接包含right join右表所有行,如果左表中某行在右表没有匹配,则结果中对应左表的部分全部为空(NULL)。
注:同样此时我们不能说结果的行数等于右表的行数。当然此处查询结果的行数等于左表数据的行数,因为左右两表此时为一对一关系。
2. 完全外连接 full join 或 full outer join
SELECT * FROM student FULL JOIN course;
idsnameidcname
1张三1足球
2李四1足球
3王五1足球
1张三2音乐
2李四2音乐
3王五2音乐
1张三4美术
2李四4美术
3王五4美术
三:内连接join或者inner join
SELECT * FROM student INNER JOIN course ONstudent.id=course.id;
idsnameidcname
1张三1足球
2李四2音乐
inner join 是比较运算符,只返回符合条件的行。
此时相当于:select * from student,course where student.id=course.id
四:交叉连接cross join
SELECT * FROM student CROSS JOIN course;
idsnameidcname
1张三1足球
2李四1足球
3王五1足球
1张三2音乐
2李四2音乐
3王五2音乐
1张三4美术
2李四4美术
3王五4美术
如果我们在此时给这条SQL加上WHERE子句的时候比如SQL:select * from student cross join course wherestudent.id=course.id
此时将返回符合条件的结果集,结果和inner join所示执行结果一样。

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

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

Full table scanning may be faster in MySQL than using indexes. Specific cases include: 1) the data volume is small; 2) when the query returns a large amount of data; 3) when the index column is not highly selective; 4) when the complex query. By analyzing query plans, optimizing indexes, avoiding over-index and regularly maintaining tables, you can make the best choices in practical applications.

Yes, MySQL can be installed on Windows 7, and although Microsoft has stopped supporting Windows 7, MySQL is still compatible with it. However, the following points should be noted during the installation process: Download the MySQL installer for Windows. Select the appropriate version of MySQL (community or enterprise). Select the appropriate installation directory and character set during the installation process. Set the root user password and keep it properly. Connect to the database for testing. Note the compatibility and security issues on Windows 7, and it is recommended to upgrade to a supported operating system.

InnoDB's full-text search capabilities are very powerful, which can significantly improve database query efficiency and ability to process large amounts of text data. 1) InnoDB implements full-text search through inverted indexing, supporting basic and advanced search queries. 2) Use MATCH and AGAINST keywords to search, support Boolean mode and phrase search. 3) Optimization methods include using word segmentation technology, periodic rebuilding of indexes and adjusting cache size to improve performance and accuracy.

The difference between clustered index and non-clustered index is: 1. Clustered index stores data rows in the index structure, which is suitable for querying by primary key and range. 2. The non-clustered index stores index key values and pointers to data rows, and is suitable for non-primary key column queries.

MySQL is an open source relational database management system. 1) Create database and tables: Use the CREATEDATABASE and CREATETABLE commands. 2) Basic operations: INSERT, UPDATE, DELETE and SELECT. 3) Advanced operations: JOIN, subquery and transaction processing. 4) Debugging skills: Check syntax, data type and permissions. 5) Optimization suggestions: Use indexes, avoid SELECT* and use transactions.

In MySQL database, the relationship between the user and the database is defined by permissions and tables. The user has a username and password to access the database. Permissions are granted through the GRANT command, while the table is created by the CREATE TABLE command. To establish a relationship between a user and a database, you need to create a database, create a user, and then grant permissions.

MySQL and MariaDB can coexist, but need to be configured with caution. The key is to allocate different port numbers and data directories to each database, and adjust parameters such as memory allocation and cache size. Connection pooling, application configuration, and version differences also need to be considered and need to be carefully tested and planned to avoid pitfalls. Running two databases simultaneously can cause performance problems in situations where resources are limited.

MySQL supports four index types: B-Tree, Hash, Full-text, and Spatial. 1.B-Tree index is suitable for equal value search, range query and sorting. 2. Hash index is suitable for equal value searches, but does not support range query and sorting. 3. Full-text index is used for full-text search and is suitable for processing large amounts of text data. 4. Spatial index is used for geospatial data query and is suitable for GIS applications.
