MySQL:NOT IN with sub select 没有按预期工作? [英] MySQL: NOT IN with sub select not working as expected?

问题描述

这是我的查询:

SELECT customer_email 
FROM   sales_flat_order 
WHERE  customer_email NOT IN (SELECT customer_email
                              FROM   sales_flat_order
                              WHERE  status != 'holded');

我的测试 customer_email [e-mail] 有 3 行状态为 holded，该邮件没有其他状态.出于某种原因，完整查询不返回匹配项.当我像这样手动填写 NOT IN 时，它起作用了，我得到了 3 行:

There are 3 rows with status holded for my test customer_email [e-mail], no other status for that mail. For some reason, the full query returns no matches. When I fill the NOT IN manually like that, it works, I get my 3 rows:

SELECT customer_email 
FROM   sales_flat_order 
WHERE  customer_email NOT IN ('whatever', 'foobar', '[e-mail]');

那么我在这里做错了什么?

So what am I doing wrong here?

小提琴:https://dbfiddle.uk/?rdbms=mysql_5.6&小提琴=f990a09528d82d7bb4e72530a5de59ec

虽然小提琴按预期工作，但我的桌子要大得多，但列的类型相同.

The fiddle works as expected though, my table is much bigger, but the columns are of the same type.

谢谢！

推荐答案

我将假设 sales_flat_order 中至少有一条记录满足条件 status != 'holded' 并且其 customer_email 为 NULL.

I will make the assumption that there is at least one record in sales_flat_order that satisfies condition status != 'holded' and whose customer_email is NULL.

(NOT) IN 对于 NULL s 是出了名的棘手，这里有一个例子.

(NOT) IN is notoriously tricky with NULLs, here is an example.

考虑以下查询:

SELECT 1 WHERE 1 NOT IN (SELECT 2 UNION ALL SELECT 3)

正如预期的那样，这会产生一个值为 1 的记录.

This yields a record with value 1, as expected.

但是，如果您将其更改为:

However if you change that to:

SELECT 1 WHERE 1 NOT IN (SELECT 2 UNION ALL SELECT NULL)

然后查询产生一个空的结果集.这是(NOT) IN 的一个众所周知的问题.出于这个原因，您通常应该避免这种语法，并使用 (NOT) EXISTS 代替.上面的查询可以改写为:

Then the query produces an empty result set. This is a well-known problem with (NOT) IN. For this reason, you should generally avoid this syntax, and use (NOT) EXISTS instead. The above query could be rewritten as:

SELECT 1 a
FROM (SELECT 1 a) t1
WHERE NOT EXISTS (
    SELECT 1
    FROM (SELECT 2 a UNION ALL SELECT NULL) t2
    WHERE t1.a = t2.a
)

DB Fiddle 演示

对于您的查询:

SELECT customer_email 
FROM sales_flat_order s
WHERE NOT EXISTS (
    SELECT 1
    FROM sales_flat_order s1
    WHERE s1.customer_email = s.customer_email AND s.status != 'holded'
);

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