这是SQL数据库的一个众所周知的限制。对于大多数数据库，必须编写... WHERE field IS NULL来测试field的NULL值。... field = NULL和... field = :param（其中:param是参数化查询的参数）都不匹配NULL值。
因此，唯一的解决方案是在查询中显式地写入field IS NULL。换句话说，你需要两个不同的查询，一个用于非空值（只有一个参数），另一个用于空值（和2个参数）。

statement = "SELECT * FROM my_table WHERE name = ? AND nickname = ?";
statement = "SELECT * FROM my_table WHERE name = ? AND nickname IS NULL";

你可以使用这个技巧（小心括号，如JBNizet所解释的）

statement = "SELECT * FROM my_table WHERE name = ? AND (nickname = ? OR nickname IS NULL)";

如果您仍然希望使用2个参数来查询NULL值。

https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html#operator_equal-to
NULL安全相等。此运算符执行与=运算符类似的相等比较，但如果两个操作数都为NULL，则返回1而不是NULL，如果一个操作数为NULL，则返回0而不是NULL。
〈=〉运算符等价于标准的SQL IS NOT DISTINCT FROM运算符。
要使null起作用，请将查询更新为：statement = "SELECT * FROM my_table WHERE name = ? AND nickname <=> ?"

@John安德森，你验证了一个答案，实际上不起作用。让我解释一下。
让我们收回经过验证的比较，并关注第二个WHERE子句：

SELECT * FROM my_table
WHERE
  name = ?
  AND (nickname = ? OR nickname IS NULL)

在? = 'something'的情况下，你有：

AND (nickname = 'something' OR nickname IS NULL)

-- which is equivalent to:
AND (NULL OR nickname IS NULL)

-- which is equivalent to:
AND (nickname IS NULL)

所以你总是得到nickname为NULL的行，不管?的值是什么（当?实际上不是NULL时的副作用）。实际上，在?不为NULL的情况下，您总是会得到NULL行和您要查找的行。

下面是一种实际操作技巧的方法（亲自测试）：

SELECT * FROM my_table
WHERE
  name = ?
  AND ((? IS NULL AND nickname IS NULL) OR (? IS NOT NULL AND nickname = ?))

2个主要病例的详细信息：

1. ?是NULL：

AND ((NULL IS NULL AND nickname IS NULL) OR (NULL IS NOT NULL AND nickname = ?))

-- which is equivalent to:
AND ((TRUE AND nickname IS NULL) OR (FALSE AND nickname = NULL))

-- which is equivalent to:
AND ((nickname IS NULL) OR (FALSE AND NULL))

-- which is equivalent to:
AND ((nickname IS NULL) OR (FALSE))

-- which is equivalent to:
AND (nickname IS NULL)

所以你得到的就是nickname为NULL的行。

1. ? = 'something'：

AND (('something' IS NULL AND nickname IS NULL) OR ('something' IS NOT NULL AND nickname = 'something'))

-- which is equivalent to:
AND ((FALSE AND nickname IS NULL) OR (TRUE AND nickname = 'something'))

-- which is equivalent to:
AND (FALSE OR (nickname = 'something'))

-- which is equivalent to:
AND (nickname = 'something')

所以你确切地得到了nickname = 'something'的行。