//easy...
var query= from a in HR_POSITION_CODE
           join b in POSITONSEQ_LEVEL_COEF_V2
           on a.position_code_id equals b.code into g
           from b in g.DefaultIfEmpty()
           select new  
           {
            a,
                Level1=b==null?"":b.Level1    
           };

------解决方案--------------------
http://msdn.microsoft.com/en-us/vcsharp/ee908647#leftouterjoin

------解决方案--------------------
探讨

select a.*,b.Level1 from HR_POSITION_CODE a  
left join POSITONSEQ_LEVEL_COEF_V2 b on a.position_code_id=b.code