Menghitung dan mengelompokkan kolom bernama sama dalam tabel berbeda (di mana klausa hanya dari satu tabel)

Coba ini:

SELECT age, COUNT(age) cnt
FROM
(
    SELECT
        T2.age 
    FROM Table1 T1
        JOIN Table2 T2
            ON T1.Id = T2.table1id
    WHERE T1.published = 1
    UNION ALL
    SELECT age
    FROM Table1
    WHERE published = 1
) A
GROUP BY age