Conditional replace/insert

[Andrei V.Loukinykh]

Hi
 If anybody know how to handle the problem, let me know please.

 I have 2 tables with the similar 4 fields.
  The first table is temporary( and small) and the second one is big.

table TMP                                  table MAIN
DATE  DN  ConvTime  DIGITS             DATE  DN  ConvTime DIGITS


 I want to add each record from the TMP into the MAIN in condition, that
there is no record in MAIN with the same combination of DATE,DN     and
DIGITS. Otherwise ( if such a combination already exists) I have to
replace existing record with the new one BUT THE FIELD ConvTime MUST BE
SUMMARISED.
That's all.
Thanks in advance.