query.sql

WITH
FirstLetters AS
(
    SELECT SUBSTRING(name, 1, 1) as letter
    FROM [NamesData]
),
LetterCounts AS
(
    SELECT letter, COUNT(letter) as letterCount
    FROM [FirstLetters]
    GROUP BY letter, TumblingWindow(second, 30)
)

SELECT * INTO [SaveGroupedData] FROM LetterCounts