SQL 不返回正确的值

Question

所以，我有一个简单的 SQL 查询，但它似乎有问题(或者我的 where-clause 写错了)，因为如果我选择特定字段( matflag )，它不会返回值特定值 ( 50 )。

查询基本上是 select from table1在table2上有一个子查询其中where-clause只是检查子查询返回的字段是否存在于 table1 中:

Select distinct  
    t1.matnum as matnum, t1.matflag as matflag, t1.factory as factory 
from 
    table1 t1, 
    (select matnum from table2 where technical_value = 'XX') t2 
where 
    t1.matnum = t2.matnum and t1.matnum = '60000000';

这将返回以下输出:

+----------+---------+---------+
| MATNUM   | MATFLAG | FACTORY |
+----------+---------+---------+
| 60000000 |         | 001000  |
| 60000000 |         | 002000  |
| 60000000 |         | 003000  |
| 60000000 |         | 004000  |
| 60000000 |         | 005000  |
+----------+---------+---------+

如果我添加 and t1.matflag != '50'到where-clause结束，整个输出消失。

Select distinct 
    t1.matnum as matnum, t1.matflag as matflag, t1.factory as factory 
from 
    table1 t1, 
    (select matnum from table2 where technical_value = 'XX') t2 
where 
    t1.matnum = t2.matnum 
    and t1.matnum = '60000000' 
    and t1.matflag != '50';

输出:

+----------+---------+---------+
| MATNUM   | MATFLAG | FACTORY |
+----------+---------+---------+

列 matflag 的附加信息:它是一个 varchar2(2 Char) 列，要么不填充任何内容，要么填充值“50”或值“10”或“20”。 现在，如果我更改 and t1.matflag != '50' 中的 where 子句至and t1.matflag is null ，输出再次正确:

Select distinct 
    t1.matnum as matnum, t1.matflag as matflag, t1.factory as factory 
from 
    table1 t1, 
    (select matnum from table2 where technical_value = 'XX') t2 
where 
    t1.matnum = t2.matnum 
    and t1.matnum = '60000000' 
    and t1.matflag is null;

因此这会返回以下输出:

+----------+---------+---------+
| MATNUM   | MATFLAG | FACTORY |
+----------+---------+---------+
| 60000000 |         | 001000  |
+----------+---------+---------+
.... and so on, have a look at the first table above

那么如果我选择 is null 它会如何返回一些内容但如果我选择 != '50' 则不会？ (旁注:将 != 更改为 <> 也没有帮助)

matflag is null怎么样？申请但是matflag != '50'不是吗？

我们运行 Oracle Database 11g 版本 11.2.0.3.0 - 64 位生产版。

Answer 1

了解如何使用正确的显式 JOIN语法:

Select distinct t1.matnum as matnum, t1.matflag as matflag, t1.factory as factory 
from table1 t1 join
     table2
     on t1.matnum = t2.matnum
where t2.technical_value = 'XX' and t1.matnum = '60000000';

然后了解NULL值以及它们如何在几乎每次比较中失败，包括 <> .

您想要的逻辑是:

where t2.technical_value = 'XX' and t1.matnum = '60000000' and
      (matflag <> '50' or matflag is null)