//LINQ:
var temp = from p in db.jj_Credentials
                       group p by p.ProfessionID into g
                       select new
                       {
                           g.Key,
                           MaxPrice = g.Max(p => p.CredentialsRankID)
                       };

//EF：

var
 
 temp1 = db.jj_Credentials.GroupBy(m => m.ProfessionID).Select(m => m.Max(o => o.CredentialsRankID));
            foreach(var p in temp1)
            {
                Response.Write(p);
            }
//補充：取最大值後取得當前完整記錄的方法，兩種寫法：

//寫法1
            var list1 = db.jj_Credentials.Where(m => m.UserId == UserContext.UserID && m.IsAudit).OrderByDescending(m => m.CredentialsRankID).ToList().GroupBy(x => x.ProfessionID)
                .SelectMany(t 
 
=> t.Select((b, i) => new { b, i }).Where(m => m.i == 0)).Select(m => m.b);
            
            foreach (var i in list1)
            {
                Response.Write(string.Format("{0} {1} {2} {3}<br>", i.CredentialsID, i.UserId, i.CredentialsRankID, i.ProfessionID));
            }

 
//寫法2
            var list = db.jj_Credentials.Where(m => m.UserId == UserContext.UserID && m.IsAudit).OrderByDescending(x => x.CredentialsRankID).ToList().GroupBy(x => x.ProfessionID)
            .Select(g => new { g, count = g.Count() })
            .SelectMany(t => t.g.Select(b => b).Zip(Enumerable.Range(1, t.count), (j, i) =>
                new
                {
                    CredentialsID = j.CredentialsID,
                    UserID = j.UserId,
                    CredentialsRankID = j.CredentialsRankID,
                    ProfessionID = j.ProfessionID,
                    rn = i
                }).Where(x => x.rn == 1));

            //列印
            foreach (var i in list)
            {
                Response.Write(string.Format("{0} {1} {2} {3}<br>", i.CredentialsID, i.UserID, i.CredentialsRankID, i.ProfessionID));
            }