seneste (maks.) dato for hvert id

du kan vælge maksimal dato for hver gruppe af id'er som

SELECT a,
    b,
    max(b) OVER (PARTITION BY a) AS c
FROM myTable
ORDER BY a,b

EDIT: en af ​​mulige løsninger på den anden(redigerede) del af spørgsmålet er

WITH cte AS (
    SELECT a,
        b,
        max(b) OVER (PARTITION BY a) AS c,
        d
    FROM myTable
)
SELECT t1.a,
    t1.b,
    t1.c,
    t1.d,
    t2.d AS e
FROM cte t1
JOIN cte t2 ON t1.a=t2.a AND t1.c=t2.b