我只需要显示最新数据（如果在B中可用），我需要显示B，否则显示A
使用UNION ALL连接表（每个表都有一个优先级），然后使用ROW_NUMBER分析函数查找每个id的第一优先级和最新日期：

SELECT *
FROM   (
  SELECT t.*,
         ROW_NUMBER() OVER (PARTITION BY id ORDER BY priority, date_column DESC) AS rn
  FROM   (
    SELECT b.id,
           b.date_column,
           b.column1,
           b.column2,
           1 AS priority
    FROM   b
    UNION ALL
    SELECT a.id,
           a.date_column,
           a.column1,
           a.column2,
           2 AS priority
    FROM   a
  ) t
)
WHERE  rn = 1;

或者，对于您的数据：

SELECT *
FROM   (
  SELECT t.*,
         ROW_NUMBER() OVER (PARTITION BY vendorid ORDER BY priority, record_no DESC)
           AS rn
  FROM   (
    SELECT u.*,
           1 AS priority
    FROM   vendor_update u
    UNION ALL
    SELECT r.*,
           2 AS priority
    FROM   vendor_registration r
  ) t
)
WHERE  rn = 1;