求 SQL多表查询，能查出重复值的语句

SELECT A,count(A) as count FROM
(SELECT A as A FROM table1
UNION ALL
SELECT B as A FROM table2
UNION ALL
SELECT C as A FROM table3)
GROUP BY username HAVING count(A) >1;

select fieid from (
    select field, ROW_NUMBER () OVER (order by t1.field) row from (
        select field from table1
        union all
        select field from table2
        union all
        select field from table3
    ) t1
) t1 where t1.row > 1