SQL to find the first occurrence of sets of data in a table

Question

Say if I have a table:

CREATE TABLE T
(
    TableDTM  TIMESTAMP  NOT NULL,
    Code      INT        NOT NULL
);

And I insert some rows:

Answer 1

PostgreSQL supports window functions, have a look at this

[EDIT] Try the following:

SELECT TableDTM, Code FROM
(
    SELECT TableDTM,
           Code,
           LAG(Code, 1, NULL) OVER (ORDER BY TableDTM) AS PrevCode
    FROM   T
)
WHERE PrevCode<>Code OR PrevCode IS NULL;