How to design the database schema to link two tables via lots of other tables

问题

Although I'm using Rails, this question is more about database design. I have several entities in my database, with the schema a bit like this: http://fishwebby.posterous.com/40423840

If I want to get a list of people and order it by surname, that's no problem. However, if I want to get a list of people, ordered by surname, enrolled in a particular group, I have to use an SQL statement that includes several joins across four tables, something like this:

SELECT group_enrolment.*, person.*
FROM person INNER JOIN member ON person.id = member.person_id
INNER JOIN enrolment ON member.id = enrolment.member_id
INNER JOIN group_enrolment ON enrolment.id = group_enrolment.enrolment_id
WHERE group_enrolment.id = 123
ORDER BY person.surname;

Although this works, it strikes me as a bit inefficient, and potentially as my schema grows, these queries could get more and more complicated.

Another option could be to join the person table to all the other tables in the query by including person_id in the other tables, then it would just be one single join, for example

SELECT group_enrolment.*, person.*
FROM person INNER JOIN group_enrolment ON group_enrolment.person_id
WHERE group_enrolment.id = 123
ORDER BY person.surname;

But this would mean that in my schema, the person table is joined to a lot of other tables. Aside from a complicated schema diagram, does anyone see any disadvantages to this?

I'd be very grateful for any comments on this - whether what I'm doing now (the many table join) or the second solution or another one that hasn't occurred to me is the best way to go.

Many thanks in advance

回答1:

Well, joins are what databases do. Having said that, you may consider propagating natural keys in your model, which would then allow you to skip over some tables in joins. Take a look at this example.

EDIT

I'm not saying that this will match your model (problem), but just for fun try similar queries on something like this:

来源：https://stackoverflow.com/questions/4749421/how-to-design-the-database-schema-to-link-two-tables-via-lots-of-other-tables