" Operators Fail to Compare with NULL in SQL?
" />
Understanding NULL Comparisons in SQL
SQL uses "!=" and "" for inequality checks. However, these operators behave unexpectedly with NULL
values.
The Issue:
Consider these SQL queries:
SELECT * FROM MyTable WHERE MyColumn != NULL; SELECT * FROM MyTable WHERE MyColumn <> NULL; SELECT * FROM MyTable WHERE MyColumn IS NOT NULL;
The first two queries return empty result sets, whereas the third correctly returns 568 rows. Why this discrepancy?
Explanation:
"!=" and "" compare values. NULL
, however, isn't a value; it signifies the absence of a value. Therefore, these operators are unsuitable for NULL
comparisons.
Use IS NULL
or IS NOT NULL
instead. These predicates specifically check for the presence or absence of a value. The third query's success stems from its correct use of IS NOT NULL
.
Key Points:
- To test for non-
NULL
values, useIS NOT NULL
. - Use "" or "!=" to compare against specific non-
NULL
values. -
NULL
cannot be considered equal to or unequal to any value, as it represents missing data.
The above is the detailed content of Why Do '!=' and '' Operators Fail to Compare with NULL in SQL?. For more information, please follow other related articles on the PHP Chinese website!

MySQLhandlesconcurrencyusingamixofrow-levelandtable-levellocking,primarilythroughInnoDB'srow-levellocking.ComparedtootherRDBMS,MySQL'sapproachisefficientformanyusecasesbutmayfacechallengeswithdeadlocksandlacksadvancedfeatureslikePostgreSQL'sSerializa

MySQLhandlestransactionseffectivelyusingtheInnoDBengine,supportingACIDpropertiessimilartoPostgreSQLandOracle.1)MySQLusesREPEATABLEREADasthedefaultisolationlevel,whichcanbeadjustedtoREADCOMMITTEDforhigh-trafficscenarios.2)Itoptimizesperformancewithabu

MySQL data types are divided into numerical, date and time, string, binary and spatial types. Selecting the correct type can optimize database performance and data storage.

Best practices include: 1) Understanding the data structure and MySQL processing methods, 2) Appropriate indexing, 3) Avoid SELECT*, 4) Using appropriate JOIN types, 5) Use subqueries with caution, 6) Analyzing queries with EXPLAIN, 7) Consider the impact of queries on server resources, 8) Maintain the database regularly. These practices can make MySQL queries not only fast, but also maintainability, scalability and resource efficiency.

MySQLisbetterforspeedandsimplicity,suitableforwebapplications;PostgreSQLexcelsincomplexdatascenarioswithrobustfeatures.MySQLisidealforquickprojectsandread-heavytasks,whilePostgreSQLispreferredforapplicationsrequiringstrictdataintegrityandadvancedSQLf

MySQL processes data replication through three modes: asynchronous, semi-synchronous and group replication. 1) Asynchronous replication performance is high but data may be lost. 2) Semi-synchronous replication improves data security but increases latency. 3) Group replication supports multi-master replication and failover, suitable for high availability requirements.

The EXPLAIN statement can be used to analyze and improve SQL query performance. 1. Execute the EXPLAIN statement to view the query plan. 2. Analyze the output results, pay attention to access type, index usage and JOIN order. 3. Create or adjust indexes based on the analysis results, optimize JOIN operations, and avoid full table scanning to improve query efficiency.

Using mysqldump for logical backup and MySQLEnterpriseBackup for hot backup are effective ways to back up MySQL databases. 1. Use mysqldump to back up the database: mysqldump-uroot-pmydatabase>mydatabase_backup.sql. 2. Use MySQLEnterpriseBackup for hot backup: mysqlbackup--user=root-password=password--backup-dir=/path/to/backupbackup. When recovering, use the corresponding life


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

Atom editor mac version download
The most popular open source editor

mPDF
mPDF is a PHP library that can generate PDF files from UTF-8 encoded HTML. The original author, Ian Back, wrote mPDF to output PDF files "on the fly" from his website and handle different languages. It is slower than original scripts like HTML2FPDF and produces larger files when using Unicode fonts, but supports CSS styles etc. and has a lot of enhancements. Supports almost all languages, including RTL (Arabic and Hebrew) and CJK (Chinese, Japanese and Korean). Supports nested block-level elements (such as P, DIV),

Dreamweaver Mac version
Visual web development tools

SublimeText3 Linux new version
SublimeText3 Linux latest version

Dreamweaver CS6
Visual web development tools
