Here are the main differences between functions and stored procedures in SQL Server:
Functions
1. Return a value: Functions must return a value, which can be a scalar value, a table, or a table-valued type.
2. Deterministic: Functions are expected to be deterministic, meaning they always return the same output given the same inputs.
3. No side effects: Functions should not have any side effects, such as modifying data or sending emails.
4. Limited to SELECT statements: Functions can only contain SELECT statements, with some exceptions for table-valued functions.
5. Can be used in queries: Functions can be used in queries, such as in the SELECT clause or in the WHERE clause.
Stored Procedures
1. Do not return a value: Stored procedures do not return a value, but they can return data through output parameters or result sets.
2. Can have side effects: Stored procedures can have side effects, such as modifying data, sending emails, or executing dynamic SQL.
3. Can contain multiple statements: Stored procedures can contain multiple statements, including INSERT, UPDATE, DELETE, and SELECT statements.
4. Can use transactions: Stored procedures can use transactions to manage data consistency and integrity.
5. Must be executed using the EXECUTE statement: Stored procedures must be executed using the EXECUTE statement, rather than being used in queries.
Choosing between Functions and Stored Procedures
1. Use functions for calculations: Use functions when you need to perform calculations or transformations on data.
2. Use stored procedures for complex operations: Use stored procedures when you need to perform complex operations, such as data modifications or transactions.
3. Consider performance: Consider the performance implications of using functions versus stored procedures. Functions can be slower than stored procedures, especially for complex calculations.