Difference between scalar, table-valued, and aggregate functions in SQL server?

Question

What is the difference between scalar-valued, table-valued, and aggregate functions in SQL server? And does calling them from a query need a different method, or do we call

Answer 1

Scalar Functions

Scalar functions (sometimes referred to as User-Defined Functions / UDFs) return a single value as a return value, not as a result set, and can be used in most places within a query or SET statement, except for the FROM clause (and maybe other places?). Also, scalar functions can be called via EXEC, just like Stored Procedures, though there are not many occasions to make use of this ability (for more details on this ability, please see my answer to the following question on DBA.StackExchange: Why scalar valued functions need execute permission rather than select?). These can be created in both T-SQL and SQLCLR.

• T-SQL (UDF):

  • Prior to SQL Server 2019: these scalar functions are typically a performance issue because they generally run for every row returned (or scanned) and always prohibit parallel execution plans.
  • Starting in SQL Server 2019: certain T-SQL scalar UDFs can be inlined, that is, have their definitions placed directly into the query such that the query does not call the UDF (similar to how iTVFs work (see below)). There are restrictions that can prevent a UDF from being inlineable (if that wasn't a word before, it is now), and UDFs that can be inlined will not always be inlined due to several factors. This feature can be disabled at the database, query, and individual UDF levels. For more information on this really cool new feature, please see: Scalar UDF Inlining (be sure to review the "requirements" section).

• SQLCLR (UDF): these scalar functions also typically run per each row returned or scanned, but there are two important benefits over T-SQL UDFs:

  • Starting in SQL Server 2012, return values can be constant-folded into the execution plan IF the UDF does not do any data access, and if it is marked IsDeterministic = true. In this case the function wouldn't run per each row.
  • SQLCLR scalar functions can work in parallel plans (