Home Database Mysql Tutorial MySQL uses numeric auxiliary tables to implement complex column changes and rows

MySQL uses numeric auxiliary tables to implement complex column changes and rows

Feb 17, 2017 pm 01:12 PM

One day, a netizen in a circle on QQ sent a message asking for help:

PropID name Amount
3 Yuanbao 2
2 Ledou 3

Can I use a SQL to write the output, that is, copy the number of items corresponding to Amount? :

PropID name
3  Yuanbao
3  Yuanbao
2  Ledou
2  Ledou
2  Ledou

[Note]:

3 Yuanbao 2
Note, the last column is 2, which means there is 2 such records will become
3 Yuanbao
3 Yuanbao
. Remove the column of the last number of records and directly use repeated rows to represent it. So according to this example, 2                                                 3           Yuanbao           will be repeated 3 times. The record is as follows:

2 乐豆
2 乐豆
2 乐豆

After some research, I couldn’t figure it out with ordinary group by sum if case when, so I had to change my mind. After getting hung up, I talked to netizen Mr.W, and he provided the idea of ​​​​a digital auxiliary table.

OK, done, the SQL is as follows:

select t1.propid,t1.name from tb t1,(select 1 as num union select 2 as num union select 3 as num) t2 where t1.amount>= t2.num order by 1;

The middle , (select 1 as num union select 2 as num union select 3 as num ) t2 part

It is best that you build another num single-column auxiliary table, from Increment from 1 to N, just give it tens of thousands, and then the associated query facilitates some self-connected SQL judgments. Select does not seem to have the function of generating numbers in a loop. As shown in the following table nums

If you see friends here and have better implementation methods, please share them.

The above is the content of MySQL using numeric auxiliary tables to implement complex column changes. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!


Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

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

Hot Tools

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

MySQL's Role: Databases in Web Applications MySQL's Role: Databases in Web Applications Apr 17, 2025 am 12:23 AM

The main role of MySQL in web applications is to store and manage data. 1.MySQL efficiently processes user information, product catalogs, transaction records and other data. 2. Through SQL query, developers can extract information from the database to generate dynamic content. 3.MySQL works based on the client-server model to ensure acceptable query speed.

How to start mysql by docker How to start mysql by docker Apr 15, 2025 pm 12:09 PM

The process of starting MySQL in Docker consists of the following steps: Pull the MySQL image to create and start the container, set the root user password, and map the port verification connection Create the database and the user grants all permissions to the database

Laravel Introduction Example Laravel Introduction Example Apr 18, 2025 pm 12:45 PM

Laravel is a PHP framework for easy building of web applications. It provides a range of powerful features including: Installation: Install the Laravel CLI globally with Composer and create applications in the project directory. Routing: Define the relationship between the URL and the handler in routes/web.php. View: Create a view in resources/views to render the application's interface. Database Integration: Provides out-of-the-box integration with databases such as MySQL and uses migration to create and modify tables. Model and Controller: The model represents the database entity and the controller processes HTTP requests.

Solve database connection problem: a practical case of using minii/db library Solve database connection problem: a practical case of using minii/db library Apr 18, 2025 am 07:09 AM

I encountered a tricky problem when developing a small application: the need to quickly integrate a lightweight database operation library. After trying multiple libraries, I found that they either have too much functionality or are not very compatible. Eventually, I found minii/db, a simplified version based on Yii2 that solved my problem perfectly.

How to install mysql in centos7 How to install mysql in centos7 Apr 14, 2025 pm 08:30 PM

The key to installing MySQL elegantly is to add the official MySQL repository. The specific steps are as follows: Download the MySQL official GPG key to prevent phishing attacks. Add MySQL repository file: rpm -Uvh https://dev.mysql.com/get/mysql80-community-release-el7-3.noarch.rpm Update yum repository cache: yum update installation MySQL: yum install mysql-server startup MySQL service: systemctl start mysqld set up booting

Laravel framework installation method Laravel framework installation method Apr 18, 2025 pm 12:54 PM

Article summary: This article provides detailed step-by-step instructions to guide readers on how to easily install the Laravel framework. Laravel is a powerful PHP framework that speeds up the development process of web applications. This tutorial covers the installation process from system requirements to configuring databases and setting up routing. By following these steps, readers can quickly and efficiently lay a solid foundation for their Laravel project.

MySQL and phpMyAdmin: Core Features and Functions MySQL and phpMyAdmin: Core Features and Functions Apr 22, 2025 am 12:12 AM

MySQL and phpMyAdmin are powerful database management tools. 1) MySQL is used to create databases and tables, and to execute DML and SQL queries. 2) phpMyAdmin provides an intuitive interface for database management, table structure management, data operations and user permission management.

Centos install mysql Centos install mysql Apr 14, 2025 pm 08:09 PM

Installing MySQL on CentOS involves the following steps: Adding the appropriate MySQL yum source. Execute the yum install mysql-server command to install the MySQL server. Use the mysql_secure_installation command to make security settings, such as setting the root user password. Customize the MySQL configuration file as needed. Tune MySQL parameters and optimize databases for performance.

See all articles