取某行数据多个数值字段最大的一个值

select 

if(field1>field2,field1,field2) as bigger,

if(if(field1>field2,field1,field2)>if(field3>field4,field3,field4),if(field1>field2,field1,field2),if(field3>field4,field3,field4)) as biggest 

from dual;