What are SQL scalar functions?

SQL scalar functions are operations that take one or more input values and return a single value. Unlike aggregate functions (which process multiple rows), scalar functions operate on individual values within a single row. These functions are commonly used to transform, format, or calculate data directly in SQL queries. Examples include UPPER() for converting text to uppercase, ROUND() for rounding numbers, and DATEADD() for modifying date values. Scalar functions are deterministic when they always return the same result for identical inputs (e.g., ROUND(5.75, 0)), or non-deterministic when results vary (e.g., GETDATE() for the current timestamp).

Developers use scalar functions in SELECT statements, WHERE clauses, or even when defining computed columns. For instance, SELECT UPPER(first_name) FROM employees; converts names to uppercase, while WHERE YEAR(order_date) = 2023 filters orders from a specific year. Scalar functions can also simplify data validation, such as using ISNULL() to replace NULL values with defaults. However, overusing them in WHERE clauses can sometimes hinder performance, especially if they prevent the use of indexes. For example, applying UPPER() to a column in a WHERE clause might require a full table scan instead of leveraging an index on that column.

Practical applications of scalar functions include formatting data for reports, cleaning inconsistent inputs, or preparing values for application logic. For example, CONCAT(first_name, ' ', last_name) combines names into a full name, while COALESCE(sales_tax, 0) ensures calculations don’t fail due to NULLs. Developers can also create custom scalar functions for repeated logic, like calculating shipping costs based on weight and distance. However, user-defined scalar functions should be kept simple to avoid performance overhead. When used appropriately, scalar functions enhance code readability and reduce the need for post-processing data in application code.