SELECT A.*, B.列2
FROM (
SELECT row1, row2 FROM 表格一
UNION ALL
SELECT 列1, 列2 FROM 表格二 WHERE 列1 NOT IN (SELECT row1 FROM 表格一)
) A
LEFT JOIN
(SELECT 列1, 列2 FROM 表格二 WHERE 列1 IN (SELECT row1 FROM 表格一)) B
ON A.row1 = B.列1;