CASE statements are a powerful mechanism in SQL code to analysis, recode, etc data. In Information Map Studio, you can utilize this CASE statement via the Expression Editor Window.

Create a new data item and click edit in the 'Expression Settings' area.
From the Categories - CASE operator folder, there are a laundry list of examples for using the CASE statement.
However, note that the functions called 'Case Statement When clause' actually do not include all the necessary components to work correctly in SQL.

Very easy fix. Just add the word 'CASE' in front of your WHEN, and then add the word 'END' after your expression. Here is an example:

CASE WHEN YEAR(<>)=YEAR(TODAY())
THEN SUM(<>)
END