SELECT D.CODE, Min(T.CODE) AS Min_CODE , Min([T].[CODE]-[D].[CODE]) AS Delt
FROM DOC AS D INNER JOIN DOC AS T ON D.CODE < T.CODE
GROUP BY D.CODE;

SELECT D.CODE, (Select Min(T.CODE) AS T_CODE  From DOC AS T WHERE D.CODE < T.CODE) AS Next,  Next - D.CODE AS DELT
FROM DOC AS D ;

SELECT D.CODE, (Select Min(T.CODE) AS T_CODE  From DOC AS T WHERE D.CODE < T.CODE) AS Next,  Next - D.CODE AS DELT
FROM DOC AS D
WHERE (Select Min(T.CODE) AS T_CODE  From DOC AS T WHERE D.CODE < T.CODE) - D.CODE> 1 ;

Option Compare Database
Option Explicit

Dim rst As DAO.Recordset
Public Function NextC(sTName As String, sIName As String, sFName As String, Code)
 'Static rst As DAO.Recordset 
On Error Resume Next
    If rst.Name <> sTName Then
        Set rst = tDBS.DataDB.OpenRecordset(sTName, dbOpenTable)
    End If
    If rst.Index <> sIName Then
        rst.Index = sIName
    End If
    rst.Seek ">", Code
    If rst.NoMatch Then
        NextC = Null
    Else
        NextC = rst.Collect(sFName)
    End If
End Function

SELECT D.Dogovor_id, T.Dogovor_id, [T].[Dogovor_id]-[D].[Dogovor_id] AS Delta
FROM Dogovor AS D INNER JOIN Dogovor AS T ON NextC("Dogovor","PrimaryKey","Dogovor_id",  D.Dogovor_id) = T.Dogovor_id
WHERE NextC("Dogovor","PrimaryKey","Dogovor_id",  D.Dogovor_id)-[D].[Dogovor_id] > 1 ;

SELECT D.CODE,
NextC("DOC","CODE","CODE",D.CODE) AS NextC,
-D.CODE+NextC AS DELT
FROM DOC AS D
WHERE
NextC("DOC","CODE","CODE",D.CODE) -  D.CODE > 1 ;

...
    NextC = Null
    If rst.NoMatch Then
         'NextC = Null 
    Else
        vBuf = rst.Collect(sFName)
        If vBuf > CODE +  1  Then
            NextC = vBuf
        End If
    End If
...

SELECT D.CODE,
 NextC("DOC","CODE","CODE",D.CODE) AS NextC,
 -D.CODE+NextC AS DELT
FROM DOCH AS D
WHERE
 Not (NextC("DOC","CODE","CODE",[D].[CODE])) Is Null
;