er图中identifying entity set和weak entity set之间的关系是唯一的（many-to- one and weak entity set total participation）吗

是的，根据ER图的many-to-one和total participation规则，一个weak entity set只能依赖于一个identifying entity set，并且identifying entity set必须参与到关系中的每一个实例中。这确保了weak entity set的完整性，以及与identifying entity set之间的唯一关系。

相关问题

Object reference not set to an instance of an object.'

This error message typically occurs in .NET programming when you try to use an object that has not been initialized. In other words, you're trying to call a method or access a property of an object that is null or does not exist.

To fix this error, you need to make sure that the object has been properly initialized before you try to use it. You can do this by checking for null values using conditional statements or by initializing the object before using it.

For example, if you have a variable named "myObject" that you want to use in your code, you can initialize it like this:

MyObject myObject = new MyObject();

This creates a new instance of the MyObject class and assigns it to the myObject variable. Now, you can safely use myObject without getting the "Object reference not set to an instance of an object" error.

If you're still having trouble identifying the source of the error, try debugging your code to see where the null reference is occurring. This will help you pinpoint the issue and make the necessary corrections.

er图实线和虚线的区别

ER图中的实线和虚线有不同的含义和用途。

实线表示标识联系（identifying relationship），而虚线表示非标识联系（non-identifying relationship）。

在标识联系中，主键是外键的一部分。通过在实体之间使用实线连接，可以将外键设为主键，从而建立标识联系。这意味着一个实体的主键值也是另一个实体的外键值。

而非标识联系则没有这样的限制。通过在实体之间使用虚线连接，可以建立非标识联系。这意味着一个实体的主键值不一定是另一个实体的外键值。

总结起来，实线表示标识联系，虚线表示非标识联系。实线连接的实体之间有主键和外键的关系，而虚线连接的实体之间没有这样的关系。