Introduction
In Transact-SQL (T-SQL), comparison operators play a crucial role in querying databases by evaluating conditions and filtering data. Among these operators, the "not equal to" operator is commonly used, with two prevalent symbols: != and <>. This tutorial explores both symbols, their origins, compatibility across various database systems, and best practices for using them in T-SQL.
Comparison Operators in SQL
ANSI Standard Compliance
The American National Standards Institute (ANSI) established a set of standards for SQL known as the SQL-92 standard. Within this framework, < > is designated as the official "not equal to" operator. This standardization ensures consistency and compatibility across different database systems that adhere to ANSI guidelines.
Proprietary Operators
While < > aligns with the ANSI standard, != is a non-standard operator introduced by Microsoft SQL Server (T-SQL) and adopted by other databases for convenience. Despite its lack of formal standardization, it functions similarly to < > in T-SQL environments.
Compatibility Across Databases
Support for Both Operators
Several database systems support both != and < >, offering flexibility based on user preference or coding standards:
- Microsoft SQL Server: Supports both operators without performance implications.
- MySQL
- Oracle
- PostgreSQL
These databases allow the use of either operator, catering to different developer preferences and historical conventions.
Exclusive Support for ANSI Operator
Some database systems strictly adhere to the ANSI standard by exclusively supporting < >:
- IBM DB2 UDB 9.5
- Microsoft Access 2010
In these environments, using != would result in syntax errors or require workarounds.
Best Practices and Recommendations
Consistency and Readability
When working within a T-SQL environment, especially in shared or collaborative projects, maintaining consistency is key:
- Choose One Operator: Decide whether to use
< >or!=consistently throughout your codebase. - Follow Team Standards: Adhere to the coding standards set by your team or organization.
Performance Considerations
For T-SQL and most databases that support both operators, there are no performance differences between using != and < >. Therefore, decisions should be based on readability, standard compliance, and personal preference rather than efficiency concerns.
Example Usage
Here is an example demonstrating the use of both operators in a simple SQL query:
-- Using < >
SELECT * FROM Employees WHERE Salary <> 50000;
-- Using !=
SELECT * FROM Employees WHERE Salary != 50000;
Both queries achieve the same result, filtering employees whose salary is not equal to 50,000.
Conclusion
In T-SQL and many other SQL-based systems, both != and < > are available for expressing "not equal to" conditions. Understanding their origins and compatibility across different databases can guide developers in choosing the appropriate operator for their projects. By adhering to ANSI standards or aligning with team preferences, developers can write clear, maintainable, and consistent SQL code.