SQL window functions provide extremely powerful and useful features that let you work with observations more easily. Different types explained here.

What Is Tsql Function? Users-defined functions are Transact-SQL or CLRS (common language runtime) routines that take parameters, perform actions, such as complex calculations, and return the results ...

DATEDIFF is a common function in the SQL Server to find the number of days between two dates. Oracle offers its own solution, although does not have the DATEDIFF function.