问题
I read my notes and I am now unsure about the keys.
My logical diagram
alt text http://files.getdropbox.com/u/175564/db/db-11.png
This table should be in line with the following physical ERD
alt text http://files.getdropbox.com/u/175564/db/db-22.png
I have only one foreign key in the table Question.
I would like to know
- should the
question-idandquestion-tagin the table Question-tag-xref, ANDquestion-idandanswerin the table Answers be also foreign keys?
回答1:
Masi - Logical models don't usually contain data type (string, int, etc) information, but do indicate if the attribute (becomes a column in the Physical model) is optional (able to be null or not).
That said, yes - the QUESTION-ID and QUESTION-TAG columns would be foreign keys in the QUESTION-TAG-XREF table. Here's a physical view of QUESTION-TAG-XREF:
QUESTION-TAGS-XREF
- QUESTION-ID (pk, fk relationship with QUESTIONS table)
- QUESTION-TAG-CODE (pk, fk relationship with QUESTION-TAG table)
Both columns are the pk to ensure you can't have duplicate tags for a given question.
You are correct that QUESTION-ID would be a foreign key in the ANSWERS table, in reference to the QUESTION table.
回答2:
Looking at both ERDs, here are the foreign keys I would want to put in place:
First ERD: Question-Tag-xref(question-id) should refer to Question(question-id). Answers(question-id) should refer to Question(question-id). Question(user-id) should refer to User(user-id).
Second ERD: Question-Tag-xref(question-id) should refer to Question(question-id). Answers(question-id) should refer to Question(question-id). Question(user-id) should refer to user-info(user-id).
I get only one foreign key in the Question table too.
In answer to your question: The column question-id in Question-tag-xref should have a foreign key reference to Question(question-id). The column question-id in the answers table should have a foreign key reference to Question(question-id)
K
来源：https://stackoverflow.com/questions/1184857/to-get-primary-and-foreign-keys-right-in-erd