PostgreSQL sequence based on another column

Question

Lets say I have a table as such:

Column   |     Type    |                        Notes
---------+------------ +----------------------------------------------         


        

Answer 1

Just a guess.

INSERT INTO TABLE (ID, SEQ, DATA)
VALUES
(
 IDVALUE,
 (SELECT max(SEQ) +1 FROM TABLE WHERE ID = IDVALUU),
 DATAVALUE
);