MySQL显然无法区分字符串

Question

我遇到了一个非常奇怪的案例：

select count(*) from shipments where user_id = 1 and pickup_id = 2返回 2

select count(*) from shipments where user_id = 1 and pickup_id = 2 and order_id = 'AAA'返回 1

select count(*) from shipments where user_id = 1 and pickup_id = 2 and order_id <> 'AAA'返回 0

到底是怎么回事？我正在使用MySQL 5.7.17

Answer 1

显然，order_id在一行中是NULL。

使用：

而不是<>

select count(*)
from shipments
where user_id = 1 and pickup_id = 2 and
      (not order_id <=> 'AAA')

<=>是NULL - 安全等式运算符（请参阅here）。 MySQL没有单NULL - 安全不等式，所以只需使用NOT或：

(order_id <> 'AAA' or order_id is null)

Answer 2

可能order_id中存在空值。 ＆LT;＆GT;不会解释这一点。

<>不是NULL容错的。但是'NOT IN'是。因此，而不是<>使用NOT IN

select count(*) from shipments where user_id = 1 and pickup_id = 2 and order_id NOT IN ('AAA')