sql - PostgreSQL: "select where"查询过滤掉具有 [null] 值的记录

Question

当我运行以下简单查询时:

SELECT gateway, customer_id
FROM gateways
where gateway = '1000056'

这是它返回的记录，显示 [null] 客户 ID:

当我添加另一个过滤器时，不再返回记录，但我认为它应该仍然返回，因为customer_id是< em>不等于21:

SELECT gateway, customer_id
FROM gateways
where gateway = '1000056'
    and customer_id != 21 --new filter!

不再返回记录:

如何过滤掉 customer_id = 21 的记录，但仍返回该 customer_id = [null] 的记录？

Answer 1

我找到了答案here 。这是该答案的直接引用:

In SQL, NULL is not equal to anything. Nor is it unequal to anything.

In other words, if I don't tell you my middle name, and you don't tell me your middle name, how can we know if our two middle names are the same name or different names? We can't know.

This often trips people up in SQL, because it's "tri-value logic." An expression can be TRUE, FALSE, or UNKNOWN. Those of us familiar with boolean algebra know that NOT TRUE is FALSE, and NOT FALSE is TRUE.

But the tricky part is that NOT UNKNOWN is still UNKNOWN.

So the solution for you is either always store a non-null string in your column, or else use an expression to account for tri-value logic

这有效:

SELECT gateway, customer_id
FROM gateways
where gateway = '1000056'
    and customer_id is distinct from 21

返回我想要的记录: