In Microsoft SQL Server, database functions are used to perform various operations on data, manipulate results, and compute values. These functions can be categorized into standard and nonstandard functions, each serving distinct purposes and characteristics. Here’s a detailed description of standard and nonstandard functions in SQL Server:

Standard Functions:

1. Purpose and Definition:
   • Standard functions, also known as built-in functions, are predefined functions provided by SQL Server and adhere to recognized SQL standards (e.g., SQL-92, SQL:1999).
   • They are designed to perform common and standard operations on data within SQL queries.
2. Examples of Standard Functions:
   • Aggregate Functions: Such as SUM, AVG, COUNT, MIN, and MAX for summarizing and calculating values across rows.
   • String Functions: Including CONCAT, LEN, UPPER, LOWER, and SUBSTRING for working with character data.
   • Date and Time Functions: Like GETDATE, DATEADD, DATEDIFF, and FORMAT for manipulating date and time values.
   • Mathematical Functions: Such as ABS, ROUND, CEILING, and FLOOR for performing mathematical operations.
   • Conversion Functions: Such as CAST and CONVERT for converting data types.
   • Logical Functions: Like CASE for conditional logic and IIF for inline IF-ELSE.
3. Portability:
   • Standard functions are more likely to be portable across different database systems, as they follow established SQL standards.
   • Code using standard functions is generally more compatible with other SQL database systems.
4. Performance Optimization:
   • SQL Server is optimized to execute standard functions efficiently.
   • The query optimizer can recognize and optimize standard functions to improve query performance.
5. Consistency and Best Practices:
   • Standard functions are recommended for consistent coding practices and adherence to industry best practices.
   • They are well-documented and supported by SQL Server documentation.

Nonstandard Functions:

1. Purpose and Definition:
   • Nonstandard functions, also known as user-defined functions (UDFs), are functions that are defined by users or developers.
   • They are created to perform specific, custom operations that may not be available through standard functions.
2. Examples of Nonstandard Functions:
   • Scalar UDFs: Functions that return a single value and can encapsulate complex calculations or business logic.
   • Table-Valued UDFs: Functions that return a result set (table) and can be used in the FROM clause of a query.
   • Inline Table-Valued UDFs: A variant of table-valued UDFs, often used for performance optimization.
3. Custom Logic and Specialized Operations:
   • Nonstandard functions are created when developers need to implement custom logic or specialized operations that cannot be achieved with standard functions alone.
   • They allow for extensibility and flexibility in SQL queries.
4. Performance Considerations:
   • Nonstandard functions may have performance implications, especially if they involve complex calculations or data manipulation.
   • Careful design and optimization of UDFs are important for maintaining good query performance.
5. Reusability:
   • Well-designed nonstandard functions can be reused across multiple queries and database objects.
   • They promote code reusability and maintainability.
6. Documentation and Testing:
   • Nonstandard functions should be thoroughly documented and tested to ensure they perform as expected and do not introduce errors into queries.
7. Security and Permissions:
   • Permissions for executing nonstandard functions must be managed, and their usage should adhere to security best practices.

In summary, both standard and nonstandard functions play essential roles in Microsoft SQL Server. Standard functions provide a common set of operations for working with data and follow SQL standards, ensuring portability and performance optimization. Nonstandard functions, or user-defined functions (UDFs), are created to address custom requirements and allow for extensibility and flexibility in SQL queries. Careful consideration of when to use standard or nonstandard functions is crucial for effective database development.