Home Database Mysql Tutorial MySQL中使用自定义变量 编写偷懒的UNION示例

MySQL中使用自定义变量 编写偷懒的UNION示例

Jun 07, 2016 pm 04:17 PM
mysql union use variable Example write customize

以下是对MySQL中使用自定义变量,编写一个UNION的示例进行了详细的介绍,需要的朋友可以过来参考下 (参考自高性能MySQL) 假设有这样的需求:写一个UNION查询,其第一个子查询作为分支先执行,如果找到了匹配的行,则不再执行第二个分支的查询。 一般来说,我

以下是对MySQL中使用自定义变量,编写一个UNION的示例进行了详细的介绍,需要的朋友可以过来参考下

 

(参考自>)
假设有这样的需求:写一个UNION查询,其第一个子查询作为分支先执行,如果找到了匹配的行,,则不再执行第二个分支的查询。

一般来说,我们可以写出这样的UNION查询:

复制代码 代码如下:


select id from users where id=123456
union all
select id from users_archived where id = 123456;


此查询可以正常运行,但是无论在users表中是否找到记录,都会到users_archived表中扫描一次;因此可能也会返回重复的记录。为了减少这种情况下不必要的开销,SQL语句可以写成这样:

复制代码 代码如下:


SELECT GREATEST(@found := -1, id) AS id, 'users' AS which_tbl
FROM users WHERE id  = 1
UNION ALL
    SELECT id, 'users_archived'
    FROM users_archived WHERE id = 1 and @found IS NULL
UNION ALL
    SELECT 1, 'reset' FROM DUAL WHERE (@found := NULL) IS NOT NLL;


上 面的查询用到了自定义变量@found,通过在结果列中做一次赋值并且放在GREATEST函数中,以避免返回额外的数据。如果第一个分支查询结果集为 NULL,那@found自然也还是NULL,因此会执行第二个分支查询。另外,为了不影响后面的遍历结果,在查询的末尾将@found重置为NULL。

另外, 返回的第二列数据是为了说明这条记录是在users表还是在users_archived表中查询得到的。

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

AI Hentai Generator

AI Hentai Generator

Generate AI Hentai for free.

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: Simple Concepts for Easy Learning MySQL: Simple Concepts for Easy Learning Apr 10, 2025 am 09:29 AM

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.

How to open phpmyadmin How to open phpmyadmin Apr 10, 2025 pm 10:51 PM

You can open phpMyAdmin through the following steps: 1. Log in to the website control panel; 2. Find and click the phpMyAdmin icon; 3. Enter MySQL credentials; 4. Click "Login".

MySQL: An Introduction to the World's Most Popular Database MySQL: An Introduction to the World's Most Popular Database Apr 12, 2025 am 12:18 AM

MySQL is an open source relational database management system, mainly used to store and retrieve data quickly and reliably. Its working principle includes client requests, query resolution, execution of queries and return results. Examples of usage include creating tables, inserting and querying data, and advanced features such as JOIN operations. Common errors involve SQL syntax, data types, and permissions, and optimization suggestions include the use of indexes, optimized queries, and partitioning of tables.

Why Use MySQL? Benefits and Advantages Why Use MySQL? Benefits and Advantages Apr 12, 2025 am 12:17 AM

MySQL is chosen for its performance, reliability, ease of use, and community support. 1.MySQL provides efficient data storage and retrieval functions, supporting multiple data types and advanced query operations. 2. Adopt client-server architecture and multiple storage engines to support transaction and query optimization. 3. Easy to use, supports a variety of operating systems and programming languages. 4. Have strong community support and provide rich resources and solutions.

How to use single threaded redis How to use single threaded redis Apr 10, 2025 pm 07:12 PM

Redis uses a single threaded architecture to provide high performance, simplicity, and consistency. It utilizes I/O multiplexing, event loops, non-blocking I/O, and shared memory to improve concurrency, but with limitations of concurrency limitations, single point of failure, and unsuitable for write-intensive workloads.

MySQL and SQL: Essential Skills for Developers MySQL and SQL: Essential Skills for Developers Apr 10, 2025 am 09:30 AM

MySQL and SQL are essential skills for developers. 1.MySQL is an open source relational database management system, and SQL is the standard language used to manage and operate databases. 2.MySQL supports multiple storage engines through efficient data storage and retrieval functions, and SQL completes complex data operations through simple statements. 3. Examples of usage include basic queries and advanced queries, such as filtering and sorting by condition. 4. Common errors include syntax errors and performance issues, which can be optimized by checking SQL statements and using EXPLAIN commands. 5. Performance optimization techniques include using indexes, avoiding full table scanning, optimizing JOIN operations and improving code readability.

MySQL's Place: Databases and Programming MySQL's Place: Databases and Programming Apr 13, 2025 am 12:18 AM

MySQL's position in databases and programming is very important. It is an open source relational database management system that is widely used in various application scenarios. 1) MySQL provides efficient data storage, organization and retrieval functions, supporting Web, mobile and enterprise-level systems. 2) It uses a client-server architecture, supports multiple storage engines and index optimization. 3) Basic usages include creating tables and inserting data, and advanced usages involve multi-table JOINs and complex queries. 4) Frequently asked questions such as SQL syntax errors and performance issues can be debugged through the EXPLAIN command and slow query log. 5) Performance optimization methods include rational use of indexes, optimized query and use of caches. Best practices include using transactions and PreparedStatemen

Monitor Redis Droplet with Redis Exporter Service Monitor Redis Droplet with Redis Exporter Service Apr 10, 2025 pm 01:36 PM

Effective monitoring of Redis databases is critical to maintaining optimal performance, identifying potential bottlenecks, and ensuring overall system reliability. Redis Exporter Service is a powerful utility designed to monitor Redis databases using Prometheus. This tutorial will guide you through the complete setup and configuration of Redis Exporter Service, ensuring you seamlessly build monitoring solutions. By studying this tutorial, you will achieve fully operational monitoring settings

See all articles