sql - NULL 的结果!= SQL 查询中的值(postgres 和 rails3)

Question

我在 Rails3 应用程序中遇到了意外的 postgres 查询问题。

我想我会通过 stackoverflow 运行它，看看互联网的大脑怎么说:)

这个结果是预期的行为(为什么？!)还是一个错误？

鉴于我的 Postgres 9.1.4 数据库中有一个表 Orders:

id        state
=====     ======
1                     <-- nil (default value)
2         'success'
3         'failure'

当我运行查询时:

Order.where('orders.state != ?', 'success').map { |order| order.id }
Order Load (3.8ms)  SELECT "orders".* FROM "orders" WHERE (orders.state != 'success')

=> [3]

我期待结果 [1, 3]。显然有 2 行满足 (!= 'success')。

为什么 nil != 'success' 在这里不是真的？ != 是否只忽略 NULL 值？应该吗？

注意:我使用以下查询生成了所需的结果:

Order.where('orders.state IS NULL OR orders.state != ?', 'success').map { |order| order.id }
Order Load (2.3ms) SELECT "orders".* FROM "orders" WHERE (orders.state IS NULL OR orders.state != 'success')

=> [1, 3]

如有任何意见，我们将不胜感激。

Answer 1

有一个IS DISTINCT FROM PostgreSQL 中的比较运算符:

Ordinary comparison operators yield null (signifying "unknown"), not true or false, when either input is null. For example, 7 = NULL yields null, as does 7 <> NULL. When this behavior is not suitable, use the IS [ NOT ] DISTINCT FROM constructs:

expression IS DISTINCT FROM expression
expression IS NOT DISTINCT FROM expression

For non-null inputs, IS DISTINCT FROM is the same as the <> operator. However, if both inputs are null it returns false, and if only one input is null it returns true. Similarly, IS NOT DISTINCT FROM is identical to = for non-null inputs, but it returns true when both inputs are null, and false when only one input is null. Thus, these constructs effectively act as though null were a normal data value, rather than "unknown".

例如，给定您的示例数据:

=> select * from orders where state is distinct from 'success';
 id |  state  
----+---------
  1 | 
  3 | failure
(2 rows)

所以你可以这样说:

Order.where('orders.state is distinct from ?', 'success').pluck(:id)

请注意，我还切换到 pluck而不是你的 map(&:id) ，这会将此 SQL 发送到数据库:

select id from orders where orders.state is distinct from 'success'

而不是 select orders.* ...使用客户端过滤器提取 id