红移意外返回一个子查询没有找到项的空值

Question

我有这样的查询返回真实没有结果：

SELECT review_id
FROM review_table
WHERE review_id NOT IN (
    SELECT DISTINCT review_id
    FROM review_migration_table
)
ORDER BY review_id

不过，我希望它返回所有review_ids不在我的迁移表（我试图找出未成功迁移所有评论）。我期待非空的结果的原因是，我知道有审查review_table不在review_migration_table：

SELECT
    review_id,
    review_id NOT IN (
        SELECT DISTINCT review_id
        FROM review_migration_table
        WHERE filter_column = 170938)
FROM review_table
WHERE
    filter_column = 170938
ORDER BY review_id ASC

 review_id | ?column?
-----------+----------
 127260864 | f
 130811274 |
(2 rows)

为什么红移在此查询的第二个记录返回“NULL”值？而且，更具体地讲，我怎么能得到我的第一次查询返回130811274（和其它评论）是在review_table但不是在review_migration_table？

Answer 1

你可以认为a NOT IN(b, c)的等同于NOT (a=b OR a=c)。我认为这是在review_migration_table表的review_id列NULL值，所以这样你NOT (a=b OR a=NULL)这要么如果FALSE或NULL，如果a=b给a<>b。这里最简单的解决办法可能是从review_id列表，e.g过滤掉NULL IN：

SELECT
    review_id,
    review_id NOT IN (
        SELECT DISTINCT review_id
        FROM review_migration_table
        WHERE filter_column = 170938 and review_id IS NOT NULL)
FROM review_table
WHERE
    filter_column = 170938
ORDER BY review_id ASC