Using Variables for calculation

Hi all, I want to calculate the time difference between two activities in an OLAP table. Based on the amount of the time difference I want to use another unit. Is it possible to create a variable which gives you a result based on the row in the OLAP? For example: Event From | Event To | Time between Post Invoice | Pay Invoice | 30 Days Post Invoice| Cancel Inovice | 2 Hours This code in the variable will be only calculated once. Therefore the variable unit give me in the second row 2 Days instead of 2 Hours. CASE WHEN MEDIAN(DATEDIFF(ss, source(WFMT_EVENTS.TIMESTAMP), target (WFMT_EVENTS.TIMESTAMP))) < 120.0 THEN sec WHEN MEDIAN(DATEDIFF(ss, source(WFMT_EVENTS.TIMESTAMP), target (WFMT_EVENTS.TIMESTAMP))) < 7200 THEN min WHEN MEDIAN(DATEDIFF(ss, source(WFMT_EVENTS.TIMESTAMP), target (WFMT_EVENTS.TIMESTAMP))) < 86400 THEN h ELSE Days END