search
HomeBackend DevelopmentPHP Tutorial求教一个mysql查询问题:为什么我的SQL语句不能覆盖所有情况?内详

mysql 查询

select count(*) from t1 where col1 > 0
339074条

select count(*) from t1 where col1 > 0 and LENGTH(col2) > 0
52308

select count(*) from t1 where col1 > 0 and LENGTH(col2) = 0
0

为什么第二条第三条查出来的结果之和和第一条查出来的结果不一样呢?

col2字段是varchar类型,长度150,存储的url地址(也可能为空或者为数字)
col1字段int类型,存储某编号 

另外:
select count(*) from t1 where col1 > 0 and col2 > 0
16484 

select count(*) from t1 where col1 > 0 and col2 = 0
32584

回复讨论(解决方案)

select count(*) from t1 where col1 > 0
339074条

select count(*) from t1 where col1 > 0 and LENGTH(col2) > 0
52308

select count(*) from t1 where col1 > 0 and LENGTH(col2) = 0
0

为什么第二条第三条查出来的结果之和和第一条查出来的结果不一样呢?

col2字段是varchar类型,长度150,存储的url地址(也可能为空或者为数字)
col1字段int类型,存储某编号 

另外:
select count(*) from t1 where col1 > 0 and col2 > 0
16484 

select count(*) from t1 where col1 > 0 and col2 = 0
32584
不一样很正常吧,col2字段是varchar类型,长度150存储的url地址(也可能为空或者为数字)
既然是字符串,那么你直接用col2 > 0 比较就已经有问题了,字符串都不能这么比较
这是其一,其二,字符串可能为空,又不是只有0或者大于0

select count(*) from t1 where col1 > 0 and col2 = ''
0

select count(*) from t1 where col1 > 0 and col2 != ''
52308

这样也不能全覆盖...

select count(*) from t1 where col1 > 0 and col2 is null 这样有没有值呢

我还是没看见你加我QQ啊,急死了- -

select count(*) from t1 where col1 > 0 and col2 is null 这样有没有值呢

有了...284786  原来是这样

多谢几位...

另外我加你QQ了你看下


select count(*) from t1 where col1 > 0 and col2 is null 这样有没有值呢

有了...284786  原来是这样

多谢几位...

另外我加你QQ了你看下 没有看见QQ消息啊。。。腾讯2B了么。我汗!!!

因为 col2 也可能为空: null
自然不属于 LENGTH(col2) = 0 范围

受教   以后mysql查字符串要考虑到is null 

Statement
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
PHP's Purpose: Building Dynamic WebsitesPHP's Purpose: Building Dynamic WebsitesApr 15, 2025 am 12:18 AM

PHP is used to build dynamic websites, and its core functions include: 1. Generate dynamic content and generate web pages in real time by connecting with the database; 2. Process user interaction and form submissions, verify inputs and respond to operations; 3. Manage sessions and user authentication to provide a personalized experience; 4. Optimize performance and follow best practices to improve website efficiency and security.

PHP: Handling Databases and Server-Side LogicPHP: Handling Databases and Server-Side LogicApr 15, 2025 am 12:15 AM

PHP uses MySQLi and PDO extensions to interact in database operations and server-side logic processing, and processes server-side logic through functions such as session management. 1) Use MySQLi or PDO to connect to the database and execute SQL queries. 2) Handle HTTP requests and user status through session management and other functions. 3) Use transactions to ensure the atomicity of database operations. 4) Prevent SQL injection, use exception handling and closing connections for debugging. 5) Optimize performance through indexing and cache, write highly readable code and perform error handling.

How do you prevent SQL Injection in PHP? (Prepared statements, PDO)How do you prevent SQL Injection in PHP? (Prepared statements, PDO)Apr 15, 2025 am 12:15 AM

Using preprocessing statements and PDO in PHP can effectively prevent SQL injection attacks. 1) Use PDO to connect to the database and set the error mode. 2) Create preprocessing statements through the prepare method and pass data using placeholders and execute methods. 3) Process query results and ensure the security and performance of the code.

PHP and Python: Code Examples and ComparisonPHP and Python: Code Examples and ComparisonApr 15, 2025 am 12:07 AM

PHP and Python have their own advantages and disadvantages, and the choice depends on project needs and personal preferences. 1.PHP is suitable for rapid development and maintenance of large-scale web applications. 2. Python dominates the field of data science and machine learning.

PHP in Action: Real-World Examples and ApplicationsPHP in Action: Real-World Examples and ApplicationsApr 14, 2025 am 12:19 AM

PHP is widely used in e-commerce, content management systems and API development. 1) E-commerce: used for shopping cart function and payment processing. 2) Content management system: used for dynamic content generation and user management. 3) API development: used for RESTful API development and API security. Through performance optimization and best practices, the efficiency and maintainability of PHP applications are improved.

PHP: Creating Interactive Web Content with EasePHP: Creating Interactive Web Content with EaseApr 14, 2025 am 12:15 AM

PHP makes it easy to create interactive web content. 1) Dynamically generate content by embedding HTML and display it in real time based on user input or database data. 2) Process form submission and generate dynamic output to ensure that htmlspecialchars is used to prevent XSS. 3) Use MySQL to create a user registration system, and use password_hash and preprocessing statements to enhance security. Mastering these techniques will improve the efficiency of web development.

PHP and Python: Comparing Two Popular Programming LanguagesPHP and Python: Comparing Two Popular Programming LanguagesApr 14, 2025 am 12:13 AM

PHP and Python each have their own advantages, and choose according to project requirements. 1.PHP is suitable for web development, especially for rapid development and maintenance of websites. 2. Python is suitable for data science, machine learning and artificial intelligence, with concise syntax and suitable for beginners.

The Enduring Relevance of PHP: Is It Still Alive?The Enduring Relevance of PHP: Is It Still Alive?Apr 14, 2025 am 12:12 AM

PHP is still dynamic and still occupies an important position in the field of modern programming. 1) PHP's simplicity and powerful community support make it widely used in web development; 2) Its flexibility and stability make it outstanding in handling web forms, database operations and file processing; 3) PHP is constantly evolving and optimizing, suitable for beginners and experienced developers.

See all articles

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 Article

R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal)
4 weeks agoBy尊渡假赌尊渡假赌尊渡假赌
R.E.P.O. Best Graphic Settings
4 weeks agoBy尊渡假赌尊渡假赌尊渡假赌
R.E.P.O. How to Fix Audio if You Can't Hear Anyone
4 weeks agoBy尊渡假赌尊渡假赌尊渡假赌
WWE 2K25: How To Unlock Everything In MyRise
1 months agoBy尊渡假赌尊渡假赌尊渡假赌

Hot Tools

Dreamweaver Mac version

Dreamweaver Mac version

Visual web development tools

SublimeText3 English version

SublimeText3 English version

Recommended: Win version, supports code prompts!

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

Atom editor mac version download

Atom editor mac version download

The most popular open source editor

SAP NetWeaver Server Adapter for Eclipse

SAP NetWeaver Server Adapter for Eclipse

Integrate Eclipse with SAP NetWeaver application server.