In T-SQL (Transact-SQL), which is Microsoft's extension to SQL, you often need to compare values in your database queries. One common type of comparison is checking for inequality. In T-SQL, there are two primary operators available for representing not equal conditions: != and <>. Here, we will explore the differences, preferences, and best practices for using these operators in your SQL queries.

Comparison of != and <>

Both != and <> are used to compare two expressions for non-equality in T-SQL. Technically, they function in the same way and are both ANSI-standard. However, <> is more traditionally seen and used in SQL. Here’s a glimpse of how both operators can be used:

1-- Using !=
2SELECT * FROM Employees WHERE DepartmentID != 5;
3
4-- Using <>
5SELECT * FROM Employees WHERE DepartmentID <> 5;

Both statements aim to retrieve all records from the Employees table where the DepartmentID is not equal to 5. Regardless of which operator you use, SQL Server processes them the same way.

Historical and Community Preferences

The <> operator is the traditional ISO standard for relational databases, and you will find it has been used historically in many tutorials and books on SQL. On the other hand, != comes from the programming realms where it is commonly used in languages like C, C++, Java, and Python. This might make != more intuitive for developers transitioning from other programming backgrounds to SQL.

Performance Considerations

Both operators perform equally well because SQL Server's query optimizer treats them the same way. There is no performance gain from using one operator over the other.

Readability and Maintainability

The choice between != and <> can also hinge on readability and maintainability of the code:

• Teams that have members from a strong programming background might find != more natural and readable.
• Conversely, teams more familiar with traditional SQL might prefer <>.

Compatibility Across Different SQL Databases

While both operators should theoretically be supported across any SQL-compliant database management system, <> is the safer bet for ensuring compatibility if your scripts might be used in different DBMSs. Some DBMS like Oracle and PostgreSQL explicitly support both, but adherence to traditional SQL with <> ensures broader compatibility.

Best Practices

• Consistency: Whichever operator you choose, maintain consistency across your codebase. Consistent usage makes the SQL scripts easier to understand and maintain.
• Team Standards: Adhere to any existing team or organizational standards. If your organization has a SQL style guide, it's best to follow it.
• Documentation: If you use both operators in different parts of your code for specific reasons, document the rationale to aid other developers.

Summary Table

Here’s a quick summary of the key points discussed:

Conclusion

In SQL Server, both != and <> are valid, perform the same, and are part of the ANSI SQL standard. The choice between them should be guided by team or personal preferences, readability, maintainability, and perhaps compatibility considerations if the code will be run on different database systems. Either way, consistency in usage is vital for clearer, more maintainable SQL code.