问题
Here is the situation:-
I have a table called Users. This contains user data for students and tutors as most of the data required is the same.
Having completed the system I am now told that the client would like to be able to assign students to tutors.
Is there a legitimate/ clean way I can create a one to many relationship within a single table, perhaps via a link table?
I've tried to think this through but whatever solution I come up with seems messy.
I would be grateful for any input.
Thanks
Phill
回答1:
Have you tried the following approach?
Make a new table, for example TutorStudent (choose a more appropriate name if needed). It should have two columns:
Tutor_IDStudent_ID
Both columns shall be the (composite) primary key, each column will be a foreign key to your Users table User_ID (I assume this is what you have).
So, if you have a tutor named Newton that has two students, Tesla and Edison, your Users table will have something like this:
- User_ID, Name
- 1, Newton
- 2, Tesla
- 3, Edison
and your TutorStudent table will have following values:
- Tutor_ID, Student_ID
- 1, 2
- 1, 3
Relatively simple and doesn't require any modifications to your existing table.
Do take care when deleting users - use the delete cascade feature of your database system or do some maintenance work afterwards so your TutorStudent table doesn't go stale when updating/removing your users.
回答2:
My ideal for the same situation
Example: one book have many category:
Basic solution: book table has recorded book information category table has recored category information ex: 100 documents book_category_relation table has single book (book_id) has category(category_id) 1 book may be have 100 category_id
Ideal solution: First calculate total your category: ex 100 document. Each category equal value 1 bit: max 31 bit so 100 category we have ceil floor(100%31) = 4 groups
category_id = 1 : 1 (1%31) <=> 000000001 group 0 = floor(1/31)
category_id = 2 : 2 (2%31)<=> 000000010 group 0 = floor(2/31)
category_id = 3 : 4 (3%31)<=> 000000100 group 0 = floor(3/31)
category_id = 4 : 8(4%31)<=> 000001000 group 0 = floor(4/31)
...........................
category_id = 31: 2^31(31%31) <=>1000..000 group 0 if moduler 31 equal zero so number group = (31/31 -1)=0;
category_id = 32: 1(32%31) <=> 0000000001 group 1 = floor(32/31)
category_id = 33: 2(33%31) <=> 0000000010 group 1 = floor(33/31)
Ok now we add 4 fields in design book table (group_0,group_1,group_2,group_3) with int(11) unsigned and add index that fields
if book has category id = n so we can the following calculate formula:
bit code = (n%31 ==0)?31: (n%31)
number group field = (n%31==0)?(n/31 -1):floor(n/31)
ex: book in category_id = 100 so:
bit code = (100%31) =7 <=>2^7 = 128,
group = floor(100%31) = 3 <=> in group_3
so if you need query all book in category_id = 100, query string is:
SELECT * FROM book WHERE group_3&128
Note: MySQL not index working if bitwise in where. But you can check in this link: Bitwise operations and indexes
来源：https://stackoverflow.com/questions/25501698/one-to-many-relationship-on-the-same-table