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.
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.
I am creating two tables in a database in MySQL just so I can play around with SQL and learn more, as I am a novice. I have read several questions on Stack relating to inserting a new row, and updating an existing row. My question is a little different, hopefully it won't be considered a dupe as none of the other answers I read gave me the full explanation I need because I think it's the auto-increment part that's confusing me. I don't think I can just go in and assign a new value for the primary keys in one of the tables with auto-increment set up, can I?
I have two tables: english_words and spanish_words. Their primary keys are respectively eng_id and span_id, and are set up to auto-increment. My hope had been to practice SQL and eventually get things set up enough so that I can practice my joins later on. For now, in english_words, I entered a duplicate row by mistake, with the ID 7. I deleted that row, and of course it now goes "6...8..." ..... and when I created my spanish_words table, I forgot all about the missing row 7. I'd hoped to keep everything very simple and aligned between the two tables until I'm ready for more complex endeavors later. Is there a way I can either:
Bump row 7 (and all subsequent rows) down by one in my spanish_words (so 7 becomes 8, 8 becomes 9, etc)
OR
Pull up everything after row 6 in english_words?
OR
Is there a better solution than either of those that you could suggest?
It's possible there's not a way. Originally I'd thought of trying to UPDATE the row 7 data in english_words or maybe insert a new row, but in my research I found an answer on Stack that said you can't insert data into a specific row in the table...and then I realized that's not going to fix anything anyway.
Do those of you more experienced with SQL have any ideas? (Aside from not making such silly mistakes anyway).
Additionally, I'm open to scrapping my tables and starting again, if there's a best-practice that I'm missing. Would setting up a foreign key to correspond between the two tables be a way to fix this? I'm pretty sure you have to do that anyway to perform the joins, but I was going to cross that bridge when I get there. What is best practice amongst database admins - set up foreign keys early on, or later when you need them?
Thanks in advance for your guidance.
A better way to set this up is to create a relation table:
CREATE TABLE translation (
eng_id int,
span_id int,
FOREIGN KEY (eng_id) REFERENCES english_words (eng_id),
FOREIGN KEY (span_id) REFERENCES spanish_words (span_id)
)
This is better than using a foreign key in the original tables, because you can't have bidirection foreign keys (you have to create the referenced row before the referencing row, so whichever table you insert into first can't have a foreign key pointing to the other one).
I have a system that has categories that are joined to events. All of these categories are simple, they have an id, and name. One thing that worries me is that when I am creating these categories, the ids should always remain the same, be static. If I deleted one, let's say "Politics" at id=1, all of those events would have an orphaned category. One solution I thought of is to just assign string ids to them, so if they do happen to get deleted, it wouldn't really matter. What kind of solution do you recommend?
From my perspective it seems like you could keep the ids and just put a constraint that doesn't allow you to delete the record, only edit them. Another, is to use string ids, but that seems like a pain, although it seems to solve the problem of worrying about the ids being messed with.
Yes, this is what foreign key constraints are for. They also allow for rules on how to handle deletes -- for example, allowing a delete to cascade through dependent records, which is of course very dangerous and not what you would want in this situation. A simple basic constraint will do.
HOWEVER!!!! This is the important thing to understand about mysql. The default mysql engine (myisam) has absolutely no support for foreign key constraints. You need to use an engine that supports them -- most commonly innodb.
If you specify a constraint when you're generating your DDL, a myisam table will accept the constraint but simply ignore it, so make sure all your related tables are setup/altered to be innodb tables before you add your constraint(s).
How do you add a constraint?
ALTER TABLE `event` ADD CONSTRAINT `category_event`
FOREIGN KEY (`category_id`) REFERENCES `category` (`category_id`);
In this example, it assumes your event table has the foreign key category_id in it, to create your linkage. After adding this constraint, if you attempt to delete a row from the category table, and an existing event row contains that key, mysql will disallow the DELETE and return an error. This is discussed in great detail here: http://dev.mysql.com/doc/refman/5.6/en/innodb-foreign-key-constraints.html
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 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 :)