SELECT LEFT(u.name, 1) AS letter, count(*) AS count FROM
    (
        SELECT name, 'Customer' AS origin FROM customer
        UNION ALL
        SELECT companyname AS name, 'Company' AS origin FROM company
    ) AS u
GROUP BY letter
ORDER BY u.name

letter | count | type
--------------------------
A        1       Customers
B        3       Both
D        1       Companies
<...>

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