Consider a simple situation in which there are 2 tables, named Users and WorkGroups.
A User's email address is the primary key in the users table.
A Workgroup_id is the primary key in the WorkGroup table.
A user can create multiple workgroups.
A user can be part of just 1 workgroup.
Under this given scenario I need to track which user created a workgroup.
What I have already done:
Have a variable named workgroup_id in the users table to know which workgroup the user belongs to. This is a foreign key to the workgroup_id in the Workgroup table.
Have a variable named user_email in the workgroup table to track which user created the workgroup. This is a foreign key to the user_email in the users table.
The problem which I am facing here is that this results in a cyclic reference between the users and the workgroups table. Since cyclic references anywhere in programming are a big no no.
How do I solve this? Is there a better design pattern which I am missing here?
EDIT:
As for whether the "circular references are a big no no" or not, conceptually they may not be but since there implementation is non universal in different databases, they still remain a valid problem. This is aggravated by the case when you have use an ORM, where the ORM support for your database limits the kind of database design you can have.
You need to allow at least one of the foreign keys to be NULL. This will allow you to create the first row in that table, leaving the foreign key empty as a placeholder. After you create the corresponding row on the other table, you update the foreign key in the first row.
You could also decide that this is OK as a permanent condition. If you create the first workgroup automatically before creating any users, that first workgroup doesn't really have a creator, so you could leave it set to NULL.
Related
I'm using MySQL and have been planning out the database structure for a system I'm building out. As I've been going along, I started to wonder if it was acceptable to have a particular foreign key constraint in many different tables. From what I understand, it would be fine, as it makes sense. But I'd like to double check.
For example, I have a users table, and I use the user_id as a foreign key for many tables, sometimes multiple times in one table. For example, I have a one-to-one relationship with a user_settings table, which of course stores the user_id. And then I have a companies table, which alone has a few references to the user_id key. In this case, I have a column that keeps track of the user that created the company in the system (created_by), a column for the main contact (main_contact, who is also a user of the system), and there might be another reference. So that alone, already has the user_id key being used as a foreign key constraint 3-4 times.
Just to add another bit of info, I have a tasks table and that of course needs to reference the user_id to keep track of who it's assigned to, and I also have another column that keeps track of the user that created the task. That would be assigned_to and created_by, respectively.
There are more tables though that reference back to that key. I might be up to 8 references already. I do believe I've designed it properly so far, but based on what I've mentioned, does this sound fine?
Your foreign key usage seems fine to me - after all, you are simply representing logical relationships between your tables.
A user within your system interacts with the data in many ways, and to define these relationships your approach is the correct one.
The key point I think is that under a lot circumstances, you won't always want (or need) to make all the joins that represent your relationships - simply the ones that you need in that context.
As per my undestanding the way you are defining is fine i.e to use a user id to many tables as foreign key.
If your line:: I have a companies table, which alone has a few references to the user_id key doesn't mean that you are using multipe user_id in same table and I know you are not.
This seems like a desirable feature but I can't seem to figure out how I would do it while the foreign key is a part of the primary key (composite key).
The table is a simple junction table for a many to many relationship referencing User.id and Access.id referencing functions a user has access to:
Column | References
user user.id
access access.id
Therefore there can be many entries for each user and each access level. To simplify things for "superusers" I wanted to have a NULL value for access which would mean they have access to every page (this is the only way I could figure how to enter a value that didn't reference a row in the access table). The problem is MySQL won't allow a NULL value as a part of the primary key.
Is there a way around the NULL issue with primary keys or is there another way to reference every row (or no rows) in a foreign key? The only other way around this I can think of would be to disable the foreign key or have to add a row for every access.id in the table. Either of which would be undesirable.
Presumably you have a superuser flag on your user table. You could UNION a Cartesian join of each superuser and the set of available access IDs into whatever query you need this for.
Depending on what you're doing, you could also just not store the access for a superuser in the database and treat them differently in code - i.e. ignore the access check once you've established them as SU. Depends on your application though.
I think NULL is allowed and you can use it as a unique combination along with user.id. But I am not sure if this is a good way to do this. I mean you can store the super user setting in a column and use it in the code than here.
I have created 2 separate tables for admins and users in my database. I want to save user and admin login details (ip address, user_agent, connection time etc) into one table. Is the only solution to create two fields one for admin ids and other for user ids in this table (like below)?
CREATE TABLE login_detail (
id int NOT NULL AUTO_INCREMENT,
admin_id int,
user_id int,
ip_address ...
...
PRIMARY KEY (id),
FOREIGN KEY (admin_id) REFERENCES admin(id) ON DELETE RESTRICT ON UPDATE RESTRICT,
FOREIGN KEY (user_id) REFERENCES user(id) ON DELETE RESTRICT ON UPDATE RESTRICT
)
If an administrator logs in, his id will be stored in admin_id and user_id will be empty. If a user logs in, his id will be stored in user_id and admin_id will be empty. What do you suggest (generally)?
I believe that ermagana understood you were converting those two tables into one table, not accessing those two tables through the new, third table. At least, that is what I assumed until I saw your response. Am I correct? If so...
In general, there is really no reason why this wouldn't all be in one table with a bit-flag indicating admin authority, as ermagana responded. I believe that would be the most common implementation, though certainly not the only option.
Your implementation using three tables, as I understand it, will require extra coding and certainly more database activity. You will need to check if the user is a user and, if not, then check if the user is an admin. Also, how are you going to ensure that the same user isn't in both tables without extra coding and database activity? At least how I understand it, it appears inefficient and error-prone.
Perhaps I don't understand it at all. If so, please clarify.
Why can't I just leave those relationships out?
What's the point of them?
I can stil run queries and treat them like it a relationship myself...
Yes, you can always leave the foreign key constraints out but then you will be responsible about the integrity of your data. If you use foreign key constraints, then you won't have to worry about the referential integrity among tables. You can read more about referential integrity from Wikipedia. I will also try to explain it with an example below.
Think of a shopping cart scenario. You have three tables: item, shopping_cart and shopping_cart_item. You can choose not to define any relationship between these tables, that's fine for any SQL solution. When user starts shopping, you create a shopping cart by adding a shopping_cart entry. As user adds items to his shopping cart, you save this information by adding rows to shopping_cart_item table.
One problem may occur at this step: If you have a buggy code that assigns incorrect shopping_cart_id's to shopping_cart_items, then you will definitely end up with incorrect data! Yes, you can have this case even with a foreign key constraint if the assigned id actually exists in the shopping_cart table. But this error will be more detectable when a foreign key exists since it would not insert shopping_cart_item record when the foreign key constraint fails.
Let's continue with the assumption that your code is not buggy and you won't have first type of referential integrity. Then suddenly a user wants to stop shopping and delete the cart and you chose to implement this case by deleting the shopping_cart and shopping_cart_item entries. Then you will have to delete entries in both tables with two separate queries. If something goes wrong after you delete shopping_cart entries, then you will again have a referential integrity problem: You will have shopping_cart_items that are not related to any shopping_cart. You will then have to introduce transaction managing, try to provide meaningful data to your business logic about the error happened in data access layer, etc..
In this type of scenario's, foreign keys can save life. You can define a foreign key constraint that will prevent insertion of any sort of incorrect data and you can define cascade operations that will automatically perform deletion of related data.
If there is anything unclear, just leave a comment and I can improve the answer.
Apart from what the others have said about why you technically want (actually: need) them:
foreign key constraints also document your model.
When looking at a model without FK constraints you have no idea which table relates to which. But with FK constraints in place you immediately see how things belong together.
You create FOREIGN KEYs to instruct the database engine to ensure that you never perform an action on the database that creates invalid records.
So, if you create a FOREIGN KEY relationship between users.id and visits.userid the engine will refuse to perform any actions that result in a userid value in visits that does not exist in users. This might be adding an unknown userid to visits, removing an id from users that already exists in visits, or updating either field to "break" the relationship.
That is why PRIMARY and FOREIGN KEYs are referred to as referential integrity constraints. The tell your database engine how to keep your data correct.
It doesn't allow you to enter an id which does not exist in another table, for example, if you have products and you keep owner Id, by creating a foreign key ton the owner id to id field of the owners table, you do not allow users to create an object record which has an owner id which does not exist in the owner table. such things are called referential intergrity.
The foreign key constraint helps you ensure referential integrity.
If you delete a row in one table, mysql can automatically delete all rows in other tables that the deleted row refers to via the foreign key. You can also make it reject the delete command.
Also when you try to insert a row, mysql can automatically create new rows in other tables, so the foreign key does not refer to nothing.
That is what referential integrity is all about.
Databases can be affected by more than just the application. Not all data changes go through the application even if they are supposed to. People change stuff directly on the database all the time. Rules that need to apply to all data all the time belong on the database. Suppose you can update the prices of your stock. That's great for updating anindividual price. But what happens when the boss decides to raise all prices by 15%. No one is going to go through and change 10,000 prices one at a time through the GUI, they are going to write a quick SQL script to do the update. Or suppose two suppliers join together to have one company and you want to change all of thie items to be the new company. Those kinds of changes happen to databases every day and they too need to follow the rules for data integrity.
New developers may not know about all the places where the foreign key relationships should exist and thus make mistakes which cause the data to be no longer useful.
Databases without foreign key constraints have close to a 100% chance of having bad data in them. Do you really want to have orders where you can't identify who the customers were?
THe FKS will prevent you from deleting a customer who has orders for instance or if you use a natural key of company_name and the name changes, all related records must be changed with the key change.
Or suppose you decide to put a new GUI together and dump the old one, then you might have to figure out all the FK relationships again (because you are using a different datalayer or ORM) and the chances are you might miss some.
It is irresponsible in the extreme to not put in FK relationships. You are risking the lifeblood of your company's business because you think it is a pain to do. I'd fire you if you suggested not using FKs because I would know I couldn't trust my company's data to you.
I'm new to foreign key constraints. I will formulate a simple example to explain my situation.
I have a table user and a table entry. In user there is a user.firstEntry which is a foreign key to entry.EntryID. In entry there is a entry.userID which is a foreign key to the user.userID table. These IDs are all auto increment values.
Are cycles like that forbidden? Then I will have to change the design?
I am not able to insert some valid entry into both tables, because the first insert already says that there's a problem with the constraints. Auto commit is off.
What shall I do?
Thanks
Bit strange design, but you can do this :
When creating a User, set firstEntry to NULL.
Insert an Entry with that user's id.
Update Users and set firstEntry to the id of the inserted entry.
Both user and entry need the other to be already created beforehand. and since either cant be created without the other, you will have this problem IF foreign constraints check is on that is.
Whatever I can understand from your question, each user seems to have multiple entries. So your table design could look like Table_User(user_id(pk), user_name etc) and the entry table could be Table_Entry(entry_id(pk), entry_whatever,...,user_id(fk to user table)) As it seems the user is independent but the entries are dependent on users.
A foreign key constraint is supposed to prevent your from adding invalid data into the foreign key column.
In most cases it will check to see if the value actually exists in the specified table. Because you have a cycle in your user and entry table, when you attempt to create a entry it will check to see if the value of entry.userID exists in the user table. It will do the same when you attempt to add a new user, it will check the entry table for the value you entered for user.firstEntry. If both user and entry are new there is no way to link the two because of your cycle. A new entry record needs an existing user and a new user record needs an existing entry. When both tables are empty I don't think you will be able to satisfy the constraint.
I would suggest keeping the foreign key to userID in the entry table (since I'm assuming entries are linked to users) and finding some other way to represent a user's first entry. Maybe an user_entry_history table or something along those lines.
DISCLAIMER - It's been awhile since I messed with Database design.