これを試して、
-- if you are using MSSQL, this query won't work because of
-- asterisk in the select clause (you need to specify all the fields)
-- in the group by clause
-- (but it will work on MySQL)
SELECT a.*, COUNT(b.Account_ID) totalOccurence
FROM Account a
LEFT JOIN Account_Position b
ON a.Account_ID = b.Account_ID
GROUP BY a.Account_ID
また
-- alternatively, you can use this.
SELECT c.*
FROM Account c
(
SELECT a.Account_ID, COUNT(b.Account_ID) totalOccurence
FROM Account a
LEFT JOIN Account_Position b
ON a.Account_ID = b.Account_ID
GROUP BY a.Account_ID
) d ON c.Account_ID = d.Account_ID