I have a website which I have been working on creating very rapidly, and now am paying back some technical debt. I have a complicated issue:
My site deals with scheduling hikes. Once you create a hike, it has many things associated with it:
a message board, list of attendees, the group it belongs to, the carpool, route, trailhead, etc.
Here is an example so you can see what I am talking about:
http://www.comehike.com/hikes/scheduled_hike.php?hike_id=172
The technical debt I am talking about is that I never made foreign keys in the DB, and now need to do a cascade delete, and I am not sure how to go about it so that I don't introduce a million bugs :)
Should I make foreign keys for all the tables now? How should I do this?
Thanks,
Alex
Check out the MySQL docs on FOREIGN KEY Constraints. Note that you'll need to be using innoDB tables.
ALTER TABLE <tablename>
ADD CONSTRAINT <fkname> FOREIGN KEY <index name>(<columns>)
REFERENCES <othertable> (<columns>)
ON DELETE CASCADE
I would suggest creating foreign keys for the tables in your DB. This will be a more robust way of dealing with the problem you are facing. You obviously understand what a foreign key imposes on the database, and how to deal with the keys.
If I was faced with this problem, I would use a graphical interface for the database if I had one (e.g. PhpMyAdmin), otherwise a quick google brings up some tutorials.
EDIT: From the linked tutorial, in a many-to-one relationship, you pace the key on the "many" table, indicating that a certain column in that table can only have values that are present in a certain column in the "one" table. Hi the link for a worked example.
When adding keys to a table that already has data, you may not be able to add the foreign key if some of the data is malformed. For example, if you have a phone number table referencing a person table (many phone numbers to one person) and you have any phone numbers with an invalid person_id (maybe person 5 was deleted and there is still a phone number with a person_id of 5) you will be unable to create the foreign key until you remove the offending phone number.
if you have not made formal foreign keys, the have you made the keys some other way that permits linking the tables or are all tables unrelated ?
If there is anyway to relate the tables then you will simply have to write a Cascading Delete code.
Otherwise its probabely a redesign or add in foreignkey fun. =))
If you have not already get yourself a copy of MySQL Workbench and redesign it from there adding in the foreign keys. This will generate the SQL code for you too.
I would go with creating foreign keys as well but if there is any reason that stops you from doing so there is another solution which is creating Triggers. You can tell triggers to basically do anything you want when an update,delete or insertion occurs to a table in the database including changing other tuples in other tables. Here are a couple of tutorials on how to create triggers:
http://www.stanford.edu/dept/itss/docs/oracle/10g/server.101/b10759/statements_7004.htm
http://msdn.microsoft.com/en-us/library/ms189799.aspx
the first one seems more direct and clearer but if none of them helps just search google for DB triggers and you're all set!
I hope this helps :)
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.
First of all, my apologies if this question is a duplicate - but I find it difficult, putting short, precise words on my problem.
I've got these entities.
The left contains several groups (like in Unix, in order to make data available to a whole group at once) and at the moment, it's always 1. The right one contains projects - and the middle one makes sure, that one group can gain access to several projects.
As you can see, there are foreign key relationships among them. Now, I want to create a new project in nmd__tree. When doing that, it returns an error:
Cannot add or update a child row: a foreign key constraint fails
(nmd.nmd__tree, CONSTRAINT FK_nmd__tree FOREIGN KEY (treeid)
REFERENCES nmd__helperusergrouphierarchy (treeidfk))
This makes sense, since the nmd_tree relies on a valid foreign key in the helper entity - but at the same time, it presents the problem, that the treeidfk isn't yet known, since it is autogenerated in nmd__tree
A solution could be to remove the relations, insert the record in nmd__tree, extract the newly written primary key (treeid) and create a record in the middle helper entity with the new id. It will work, but is really not very elegant. Also, removed relations presents other, potential problems.
My intentions are to create a query, that deals with this problem by creating both records at once. I know, it isn't possible to make a double insert and found this suggestion (my version doesn't write any records), as well as an article, suggesting stored procedures, which I don't see should make a difference
I would really appreciate a push in the right direction, please.
It seems you've got your constraints defined in the wrong direction; The middle table should have two foreign key constraints not the two end tables. That way, you can insert records in the two end tables and then link them up using the middle table.
I went over a legacy database and found a couple of foreign keys that reference a column to itself. The referenced column is the primary key column.
ALTER TABLE [SchemaName].[TableName] WITH CHECK ADD
CONSTRAINT [FK_TableName_TableName] FOREIGN KEY([Id])
REFERENCES [SchemaName].[TableName] ([Id])
What is the meaning of it?
ALTER TABLE [SchemaName].[TableName] WITH CHECK ADD
CONSTRAINT [FK_TableName_TableName] FOREIGN KEY([Id])
REFERENCES [SchemaName].[TableName] ([Id])
This foreign key is completely redundant and pointless just delete it. It can never be violated as a row matches itself validating the constraint.
In a hierarchical table the relationship would be between two different columns (e.g. Id and ParentId)
As for why it may have been created quite likely through use of the visual designer if you right click the "Keys" node in object explorer and choose "New Foreign Key" then close the dialogue box without deleting the created foreign key and then make some other changes in the opened table designer and save it will create this sort of redundant constraint.
In some cases this is a preferred way to reduce redundancy in your model. In using the self referencing foreign key (as shown in you example) you create a hierarchical relationship between rows in your table. Pay attention to what happens when you delete a row from the table, cascading on delete might remove rows you still want.
Using these sort of keys moves some of the data validation to the DB model as opposed to making this a responsibility of the program/programmer. Some outfits prefer this way of doing things. I prefer to make sure programs and programmers are responsible - data models can be hard to refactor and upgrade in production environments.
I have a table named USERS with user_id as primary key and user_name.
I have another table USERS_ACT with user_act_id primary key, user_act_user_id and another 2 columns.
I need user_act_user_id to be foreign key in USERS? How can I achieve this?
This is my first day in SQL so please be kind to explain if what I ask is wrong.
let's assume you are not the DB admin and you just want to get all the active users' names ;))
select users.user_name
from users
join users_act on users.user_id = users_act.user_act_user_id
Without referencial integrity it's up to you to make it work, there's no "magic" around it.
Populate your user_act_user_id with a pk-value from USERS and there you have it.
You may want to add constraints, but that may not be what you're asking for,
http://msdn.microsoft.com/en-us/library/ms175464.aspx
In short, they keep the keys between tables in good shape.
Assuming you are using InnoDB (which is the only engine that supports foreign keys):
ALTER TABLE users_act
ADD CONSTRAINT fk_users_act_users
FOREIGN KEY (user_act_user_id)
REFERENCES users (user_id);
It depends on your DB Type if MySql even supports foreign keys. For example you can use foreign keys with InnoDB format but not with MyIsam format.
When working with MySql i personally prefer working with MyIsam and do most of the checking about integrity while programming.
In general you can just add user_act_user_id in your table USERS but not mark it as any key. After that you can simple use a JOIN, but ofc the referencial integrity is not given so have to write your own "trigger" on programming site if you want f.e. to automaticly delete data belonging to a user in the other table. Otherwise you have to use constraints or triggers, but this might be not that easy when just started with SQL.
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.