comparing a variable in when oracle sql

I want to use a variable in when clause of oracle sql like below DEFINE balancing_id = 0--'16493'--null; ON A.BALANCING_ID = CASE WHEN &balancing_id is null-- &balancing_id = ''--balancing_id>0 THEN B.BALANCING_ID ELSE &balancing_id END But I get the error below : ORA-00920: invalid relational operator 00920. 00000 - "invalid relational operator" *Cause: *Action: this line: >WHEN &balancing_id is null-- &balancing_id = ''--balancing_id>0 UPDATE: > WHEN 1 = 1 works as expected How to use variable like this?