关系数据库设计问题

Question

While designing a relational database I got the questoin in my head if the design I made could be made easier. The design consists of 4 tables: (simplified)

• location
• office
• school
• group of schools

PK = primary key, FK = foreign key

Lets say we want to get details about a location. If the location is a school we want to see the name, amount of students and the name of the group of schools its part of. If it is a office we only want to see the name of the office. With the current design this is impossible to do in 1 query (with my knowledge).

question: Is there a better way to design this database so I can get the needed details about a location in 1 query.

Although there may be a way to get these details in 1 query I am more interested in enhancing the database design.

Thanks in advance, Knarfi

ANSWER: I finally found the term that describes my problem: Polymorphic Association. Especially this question gave me a good answer.

Answer 1

You can try something like this

SELECT location.*,school.*,office.*
FROM location
LEFT JOIN school ON school.locationID=location.locationID
LEFT JOIN office ON office.locationID=location.locationID

In result you will have NULL values for school fields if there will be office and vice versa. But you have to check this in your application and make decision there - database sends you all data.

Answer 2

You can merge School and Office to one Entity, say: Organization with the following attributes:

• Organization_ID (PM)
• Type (1=School, 2=Office)
• Location_ID (FK)
• Group_ID (FK) (it can have values only for the schools, unless you want to add group info for offices too ex: Accounting, Sales, etc in table Groups)
• Name
• NumOfMembers (it can be applied to both schools and offices)

About your last question: " Although there may be a way to get these details in 1 query I am more interested in enhancing the database design. " I must say that this is not a design question but an implementation one. Remember that it is a very bad practice to change your design according to your implementation needs...

In any case you can use ONE query in both cases (yours and mine) see other people answers. tIp: try to use views... they are usefull in such cases.

Answer 3

The database design doesn't restrict a location from having both a school and office related to it at the same time. (Could be multiple school and/or multiple office.)

There's several possible ways to retrieve information related to location .

Here's one approach:

SELECT 'school' AS source
     , s.name
     , s.students
     , g.name AS school_group_name
  FROM location sl
  JOIN school s
    ON s.location_id = sl.id
  LEFT
  JOIN group_of_schools g
    ON g.id = s.group_id
 UNION ALL
SELECT 'office' AS source
     , o.name
     , NULL
     , NULL
  FROM location ol
  JOIN office o
    ON o.location_id = ol.id

If you need to restrict to a specific set of location, you'd need to add WHERE clause on each SELECT .

Add an ORDER BY if you want the rows returned in a particular order.