我们可以使用一个IN子句，并且假设表2的id必须出现在表1中：

SELECT id, A, B, C
FROM t1
UNION ALL
SELECT id, A, B, C
FROM t2
WHERE t2.id IN (SELECT id FROM t1);

试用：db<>fiddle

比如：

SELECT id, a, b, c
from t1
union all
SELECT id, a, b, c
from t2
where exists (select *
              from t1
              where t1.id = t2.id);

我们可以在窗口函数的帮助下尝试下面的联合方法：

WITH cte AS (
    SELECT id, A, B, C, 1 AS src FROM t1
    UNION ALL
    SELECT id, A, B, C, 2 FROM t2
),
cte2 AS (
    SELECT *, MIN(src) OVER (PARTITION BY id) AS min_src,
              MAX(src) OVER (PARTITION BY id) AS max_src
    FROM cte
)

SELECT id, A, B, C
FROM cte2
WHERE src = 1 OR min_src <> max_src
ORDER BY id, src;

大概是这样的

SELECT t1.id, t1.A, t1.B, t1.C
FROM t1
FULL OUTER JOIN t2 ON t1.id = t2.id
WHERE t1.id IS NOT NULL