SELECT [t0].[MemberID], [t0].[MemberName]
FROM [dbo].[MemberInfo] AS [t0]
LEFT OUTER JOIN [dbo].[MemberSort] AS [t1] ON ([t1].[MemberType] =0) AND 
([t0].[MemberID] = [t1].[MemberID])
ORDER BY 
    (CASE 
        WHEN [t1].[SortNumber] IS NOT NULL THEN [t1].[SortNumber]
        ELSE 9999
     END)

var data = from m in db.MemberInfo
    join s in db.MemberSort on m.MemberID equals s.MemberID && 
    s.MemberType == 0
    into x
    from cx in x.DefaultIfEmpty()
    orderby cx.SortNumber.HasValue ? cx.SortNumber.Value : 9999
    select m;

var data = from m in db.MemberInfo
    join s in db.MemberSort on m.MemberID equals s.MemberID
    into x
    from cx in x.Where(d => d.MemberType == 0).DefaultIfEmpty()
    orderby cx.SortNumber.HasValue ? cx.SortNumber.Value : 9999
    select m;