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

Try this:

SELECT MIN(TableDTM) TableDTM, Code
FROM
(
    SELECT T1.TableDTM, T1.Code, MIN(T2.TableDTM) XTableDTM
    FROM T T1
    LEFT JOIN T T2
    ON T1.TableDTM <= T2.TableDTM
    AND T1.Code <> T2.Code
    GROUP BY T1.TableDTM, T1.Code
) X
GROUP BY XTableDTM, Code
ORDER BY 1;