如何把select语句中得到的null值替换成其它字符

select * from V_opration

number     opration     pubname   proname  expname
-----------------------------------------------------------
1                Add         admin          null          null
2                del           admin          null           null
3                res           null              IBM         null
4                give         null               null           A1

把后面三列合并:
select number,opration,coalesce(pubname,'')+coalesce(proname,'')+coalesce(expname,'')
as OPNAME  from V_opration