IIF and CHOOSE Functions

The IFF and CHOOSE functions are non-standard T-SQL functions introduced in SQL Server 2012. These functions were added to help simplify migration from Microsoft Access.

IFF Function
Think of this as an If-Else function.

Example:

DECLARE
@a SMALLINT = 7,
@b SMALLINT = 19;
SELECT IIF(@a < @b, 'true', 'false'); -- Returns 'true'
GO

This is similar to CASE statement like below:
DECLARE
@a SMALLINT = 7,
@b SMALLINT = 19;
SELECT CASE WHEN @a < @b
THEN 'true'
ELSE 'false'
END -- Returns 'true'

Although IIF may seem like less code to write in most scenarios. It's still recommended to use standard functions unless you are migrating from Microsoft Access.
Link to Microsoft Documentation: https://msdn.microsoft.com/en-us/library/hh213574.aspx

CHOOSE Function
The CHOOSE function is straightforward. It will choose from a list of options based on the index position that you provided in the first parameter.

Example:

SELECT CHOOSE(2, 'eric', 'kyle', 'stan', 'kenny'); -- Returns 'kyle'
SELECT CHOOSE(4, 'eric', 'kyle', 'stan', 'kenny'); -- Returns 'kenny'
SELECT CHOOSE(0, 'eric', 'kyle', 'stan', 'kenny'); -- Returns NULL 

Link to Microsoft Documentation: https://msdn.microsoft.com/en-us/library/hh213019.aspx