Choosing the Right Operator: != or <> for Non-Equality in T-SQL

Functionality:

• Both != and <> perform the same task: they check if two expressions are not equal.
• They return TRUE if the expressions are not equal, and FALSE otherwise.

Standardization:

• <> is the standard way to write the not equal operator according to the SQL-92 standard.
• != is a non-standard operator specific to T-SQL (used in Microsoft SQL Server).

Choosing Between Them:

• Portability: If you plan to port your T-SQL code to other database systems that adhere strictly to the SQL standard, using <> is recommended for better compatibility.
• Readability: Some developers may prefer <> for its cleaner visual appearance, especially if they're familiar with other SQL dialects.
• Consistency: If you're working on a large codebase with existing conventions, it's best to maintain consistency within that project.

Performance:

• There's no inherent performance difference between != and <> in T-SQL. Both are optimized by the SQL Server query engine.

In Summary:

• For portability and adherence to the SQL standard, use <>.
• If project consistency or personal preference dictates !=, it's functionally equivalent within T-SQL.

Additional Considerations:

• When working with NULL values, both operators return NULL if either operand is NULL. Consider using IS NULL or IS NOT NULL for explicit NULL checks.

By understanding these points, you can make informed decisions about using != or <> in your T-SQL code.

Selecting Customers Not Named 'John Smith'

-- Using !=
SELECT *
FROM Customers
WHERE CustomerName != 'John Smith';

-- Using <> (equivalent result)
SELECT *
FROM Customers
WHERE CustomerName <> 'John Smith';

Finding Orders with a Status Not Equal to 'Pending'

-- Using !=
SELECT *
FROM Orders
WHERE OrderStatus != 'Pending';

-- Using <> (equivalent result)
SELECT *
FROM Orders
WHERE OrderStatus <> 'Pending';

Checking for NULL Values (both operators return NULL here)

-- Using !=
SELECT *
FROM Products
WHERE Price != NULL;  -- This might not be what you want!

-- Using <> (equivalent result)
SELECT *
FROM Products
WHERE Price <> NULL;  -- This might not be what you want!

Alternative for NULL checks (using IS NULL)

SELECT *
FROM Products
WHERE Price IS NOT NULL;

Remember, for explicit NULL checks, use IS NULL or IS NOT NULL instead of relying on the behavior of != or <> with NULL values.

1. Using NOT with the Equal Operator (=):

   SELECT *
   FROM Customers
   WHERE NOT CustomerName = 'John Smith';
   

   This approach achieves the same result as != or <> by negating the equality condition. It can be slightly less readable for some developers, but it can be useful if you need to emphasize the negation aspect of the comparison.

2. CASE Expressions (for Complex Logic):

   If you're dealing with more complex comparisons that involve multiple conditions or actions based on the comparison result, you can use a CASE expression. This allows for a more structured approach:

   SELECT *
   FROM Orders
   CASE WHEN OrderStatus = 'Pending' THEN 'Pending order'
        ELSE 'Non-pending order'
   END AS OrderStatusDescription;
   

   Here, the CASE expression checks the OrderStatus and assigns different descriptions based on whether it's equal to 'Pending' or not. This can be helpful for data transformation or creating custom output based on comparison results.

Important Note:

While these approaches can be used in certain situations, they are generally not considered better practices than using != or <> for simple not equal comparisons. The standard operators are more concise and efficient for straightforward checks.