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.
Related
Let's say my database is for ecommerce store. Database holds records of users and orders. I have 2 tables 'users' and 'orders'.
'orders' table have userId column(foreign key) that reference to 'users' table id column.
When I want to delete a user- it throws an error because the 'orders' table have a record referencing this user's id. So how should I handle this?
I found 3 ways to overcome this.
not use foreign keys
use 'ON DELETE CASCADE' so when i delete user from 'users' table it will delete related order records of that user automaticaly.(is it a good idea?)
delete all child records and then delete parent record.
What is the best way?
For the simple question of how do you delete a record and it's references...
Foreign keys are critical for the integrity of your database. Without foreign keys you easily wind up with records that refer to objects which no longer exists. Don't remove them.
Manually deleting the referencing rows is error prone and will break when you change the schema.
on delete cascade allows you to have referential integrity, and also be able to delete records.
The comments got into the larger question of whether deleting user and order records are a good idea. There is not enough information in your question to know what is best. That would be another question.
However, even if you decide to use a status field to set users and orders as inactive (a timestamp, not a flag, because you'll want to know when the user was deactivated), you still want to set up the tables with on delete cascade so when you eventually do delete inactive records (for example, perhaps an annual cleanup, or for testing, or due to a mistake) the delete will work.
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.
I've almost completed my very complex project, then realized I had one issue still hanging me up. How do you (or is it even possible) to have the foreign key auto update (to reflect the parent) for each child table?
I guess I should mention the main site is built through Joomla. Everything I will be referring to in this post resides outside of Joomla but on the same domain, with the exception to the user profiles.
For instance...
I have table dataCompany that stores the company information. This table will be a child table to the userProfile table. Now, userProfile table has an auto-incremented column (user_id) and I created a column by the same name in dataCompany (but not auto-incremented). I want the user_id in dataCompany to auto fill to match that from userProfile.
The user will be logged in when entering data into dataCompany, which I thought would make it easier to autofill the user_id foreign constraint. However, I still get the invalid error.
Perhaps I'm missing something, but I was under the impression foreign keys were the only way to hold separate tables together.
EDIT:
I'm pretty sure I didn't explain what I'm doing very well.... so here's another try.
I have a website. The user will sign up for said website. That creates user_id in mysql database. I have a form the user will fill out, and the information will be stored in dataCompany. This table has primary key of companyID but also has column user_id. I want the data the user inserts into dataCompany to be associated with the user data tied together by user_id. Now, when the user signs up the user_id is auto-incremented. I can't have the user putting in their user_id when filling out their company information (as they don't know what it is)... that's where the 'auto update foreign key' comes from. I just want it to replicate what is already in the parent table (userData).
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.
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.