Dear experts, I have columns coming from two different datasets which also resides in two different tables in the same layout,now i need the difference of that columns. Example: sql>select (select mag from one_to_many o, targets16 t where o.new_targetid=t.id) - (select mag from one_to_many o,targets16 t where o.old_uniqueid=t.id) ; cardinality violation (22>1) p.s. these two tables have both 22 rows. But if I subtract on single values, it works: select (select avg(mag) from mag1) - (select avg(mag) from mag2); +--------------------------+ | sql_sub_L1 | +==========================+ | -0.015908087452235264 | +--------------------------+ 1 tuple (4.486ms) can any one help me out? Thanks In Advance !!!! Meng