Home Database Mysql Tutorial 【mysql】关于子查询的一个例子

【mysql】关于子查询的一个例子

Jun 07, 2016 pm 03:07 PM
linux mysql example about Inquire Enter

欢迎进入Linux社区论坛,与200万技术人员互动交流 >>进入 假设表my_tbl包含三个字段a,b,c;现在需要查询表中列a的每个不同值下的列b为最小值的记录量。 比如表记录为: a b c 1 3 'cd' 2 3 'nhd' 1 5 'bg' 2 6 'cds' 1 7 'kiy' 3 7 'vsd' 3 8 'ndf' 希望得到

欢迎进入Linux社区论坛,与200万技术人员互动交流 >>进入

  假设表my_tbl包含三个字段a,b,c;现在需要查询表中列a的每个不同值下的列b为最小值的记录量。

  比如表记录为:

  a  b  c

  1  3  'cd'

  2  3  'nhd'

  1  5  'bg'

  2  6  'cds'

  1  7  'kiy'

  3  7  'vsd'

  3  8  'ndf'

  希望得到结果为:

  a  b  c

  1  3  'cd'

  2  3  'nhd'

  3  7  'vsd'

  (1)

  其中一个做法:先查出每个a值下的b最小值,然后根据这些最小值去查询符合要求的所有记录。

  查询符合最小b值的sql写法如下:

  select A.* from my_tbl as A where A.b=(select min(b) from my_tbl as B where B.a=A.a);

  由于是嵌套查询和取交集,80万条记录情况下竟然用一个小时也没把中间结果算出来(我真怀疑是自己哪里写错了);后面求记录量就免谈了。

  (2)

  上面的方法是个灾难, 只能弃用了。

  具体逻辑为:先按列a,b分组,然后选择每组中列b值最小的记录,生成结果集。

  sql语句写法如下:

  select a,b,c,count(a) from (select a,b,c from my_tbl group by a,b) as A group by a;

  执行查询后,时间竟只用了1.1秒。

  再一次证明,sql的查询策略的不同能直接导致性能上的巨大差异。

【mysql】关于子查询的一个例子

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.

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.

How to run java code in notepad How to run java code in notepad Apr 16, 2025 pm 07:39 PM

Although Notepad cannot run Java code directly, it can be achieved by using other tools: using the command line compiler (javac) to generate a bytecode file (filename.class). Use the Java interpreter (java) to interpret bytecode, execute the code, and output the result.

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 check the warehouse address of git How to check the warehouse address of git Apr 17, 2025 pm 01:54 PM

To view the Git repository address, perform the following steps: 1. Open the command line and navigate to the repository directory; 2. Run the "git remote -v" command; 3. View the repository name in the output and its corresponding address.

MySQL vs. Other Programming Languages: A Comparison MySQL vs. Other Programming Languages: A Comparison Apr 19, 2025 am 12:22 AM

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.

laravel installation code laravel installation code Apr 18, 2025 pm 12:30 PM

To install Laravel, follow these steps in sequence: Install Composer (for macOS/Linux and Windows) Install Laravel Installer Create a new project Start Service Access Application (URL: http://127.0.0.1:8000) Set up the database connection (if required)

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.

See all articles