使用NOT IN的MS Access SQL

Question

I am working in MS Access 2013 with these two tables.

t1:       t2:
ID        ID   F1   F2
1              a    a
2         1    b    a
3         2    b    a

I know it is not the most ideal way to do this, but I wrote this query to pull all IDs that dont exist in another table.

Select [ID] from table1 WHERE [ID] NOT IN (SELECT ID FROM table2)

The query should return 3, but for some reason this table gives me no results. When I change the query to this, it gives me 3 as the result.

Select [ID] from table1 WHERE [ID] NOT IN (SELECT [ID] FROM table2 WHERE [ID] IS NOT NULL)

I am unsure to why the logic of why the second query works, but the first one returns no results. When I run the nested query (SELECT [ID] FROM table2) it does not return 3 for either query, so both queries should work. Can anyone explain why the first query does not return anything?

Answer 1

I strongly recommend that you simply forget using not in with subqueries. Instead, use not exists :

Select [ID]
from table1 as t1
where NOT EXISTS (SELECT 1 FROM table2 as t2 WHERE t2.id = t1.id);

As you have learned, not in has the wrong semantics (ie behavior) when the subquery returns NULL . Instead of trying to make it work, just use NOT EXISTS . An added bonus is that this readily extends to multiple columns and should use an index if available.

Answer 2

无法评估Null值，因此，当查询尝试查看3是否=为null它不知道如何检查，因此不返回任何内容。

Answer 3

这是因为In的工作方式（当然不是Not In）在此问题的答案中有详细说明： SQL“选择不在子查询中的位置”不会返回任何结果

Answer 4

If you insist on using NOT IN, use this (assuming ID can't be negative in table2):

Select ID
from table1 
WHERE ID
NOT IN 
(
SELECT Nz(ID,-1) 
FROM table2
)

Nz converts a Null value to the provided parameter. If no parameter is provided, Nz converts to zero or a zero-length string.