CODEWITHSUNDEEP
mysqlsqldatabasedatabase-designforeign-keys
user489041
user489041

Reputation: 28312

SQL Table Design

I have a table called Project. For simplicity, lets assume it has one column called projectId.

Projects
---------
projectId    PK

I have another table. It is called SubProjects. It has two columns, one is called projectId, and the other is called subProjectId. Both columns make up the primary key.

Sub Projects
--------
projectId       PK
subProjectId    PK

They way this works is you have a bunch of projects listed in the projects table. Some of these projects can be sub projects of projects. For example, if the projects table had 3 rows like so

Projects
-------
projectId
    1
    2
    3

Projects 2 and 3 could be considered sub projects of project 1.This is where the SubProject table comes in. To represent this, Subprojects would look like this

Sub Projects
--------
1 ,2
1 ,3

Meaning that projects 2 and projects 3 are sub projects of project 1.

I am having issues creating a model for this. In the SubProjects table, I need both projectId and subProjectId to reference a projectId in the Projects table. But, the key thing is that they could be referencing different projects. I supose I should do this using foregin keys? Does this make sense? Is this allowed? Is there a better way to do this?

I am using MySql and MySql workbench, but cant seem to pull this off. Any ideas?

EDIT A project can be a sub project of multiple tables.

Upvotes: 0

Views: 609

Answers (5)

Farhan
Farhan

Reputation: 2575

Why don't you add a new field in table Projects, named as ParentProjectID. For projects, it will be null, and for subprojects, it will have its parent projectID there.

Upvotes: 0

user1429080
user1429080

Reputation: 9166

You can add foreign key constraints to both columns in "Sub Projects" referring back to the ProjectId column in the "Projects" table. Doing so will not force both columns to have the same value on the row in "Sub Projects". When you insert a new row to "Sub Projects" (lets say 2,3), the server will check:

  • That there is a row in the Projects table which has a ProjectId value 2
  • That there is a row in the Projects table which has a ProjectId value 3
  • The there is not already an identical row in "Sub Projects"

This kind of model will work but it will not guarantee that the project hierarchy is "sane". There is nothing in the database itself to stop you from inserting both (2,3) and (3,2) into the "Sub Projects" table. Or (1,1) for that matter. So you will have to enforce that in your application logic instead.

Upvotes: 0

Leasure
Leasure

Reputation: 31

How you considered creating one table with a hierarchy? Using your example it would be something like

Projects
----------

ProjectID | ParentID
----------|----------
   1      |   NULL
   2      |   1
   3      |   1

Upvotes: 0

hannebaumsaway
hannebaumsaway

Reputation: 2744

I would create one table with hierarchy using columns, such as:

project_id    parent_project_id
-------------------------------
1             null
2             1
3             1

Then you can just say if a project's parent_project_id is null, it is not a sub-project of any other project. Otherwise, the project is a sub-project of the project listed in parent_project_id.

Upvotes: 0

Ask Bjørn Hansen
Ask Bjørn Hansen

Reputation: 6953

Doing it like that should be fine. Can you be more specific about why you can't set it up?

If a project is only a sub-project of one project, another way to do it would be to add a "parent project" column to the projects table, with a FK pointing to "itself". I think your queries most likely will end up being simpler if you do it that way.

Upvotes: 1

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