在SQL中，您无法比较空值，即，即使字段中的值为空，EngSerialNum = null始终求值为false.

In SQL you can't compare null values, i.e. EngSerialNum = null always evaluates to false, even if the value in the field is null.

您可以动态创建查询，以便使用is null来匹配空值，或者可以使用如下表达式:

Either you can create the query dynamically so that you use is null to match the null values, or you can use an expression like this:

((EngSerialNum is null and @EngSerialNo is null) or EngSerialNum = @EngSerialNo)