What is a Named Query? In what scenarios do you use it?

A Named Query is a SQL query/expression in your DSV which acts as a Table. It is used to combine data from one or more Table(s)/View(s) from the underlying data source without requiring any schematic changes to the underlying data source Table(s)/View(s).

 
The SQL Query used in the Named Query should conform to the underlying data source dialect. For example, if the underlying data source is SQL Server, then it should conform to T-SQL, If is it Oracle, then it should conform to PL/SQL, etc...

 
Named Queries are used in various scenarios, following are some of the common scenarios:
Combining data from multiple Tables/Views from the underlying data source by using either simple or complex join operations.

 
Adding filter conditions for filtering out unwanted data or selecting the required data (limiting the data).Pretty much everything that can be done using a Named Calculation can also be done using a Named Query.