is it possible to have two foreign key in table from same table
example post table has user_id and username from table user
--------------------------------
|table user | table post |
|-------------|----------------|
|user_id | post_id |
|username | post_title |
|password | post_content |
|email | user_id Fk |
| | username Fk |
--------------------------------
Technically fine.
But keep in mind that the structure listed above could lead to some strange situations.
If a user is allowed to change his own username (which is a separate discussion as to whether that is advisable or not) then you could end up with some scenarios where either a user would not be able to change his name (because then Foreign Key integrity would be broken with a post that refers to his name) or a user could try to hijack another user's posts by changing name to match.
All these problems can be prevented easily enough, but as a general rule I think is better to stick to a single Foreign Key and, generally, to use a number (like user_id) instead of text.
#Alan mentions the idea of two fields in a table referencing the same Foreign Key field of another table - that is quite common and generally OK.
yes, it is possible to use two foreign keys in same table .
But in your case you may not need 'username' as foreign key. because 'user_id' can be used to select username .
Related
This should be a simple question I think, but is it OK to have NULL foreign keys?
I guess to elaborate, let's say I'm making a database for users and different types of users require different data sets... what would be the best practice to design this database?
This was my thought, as a rough example: (am i correct or way off?)
"users":
id | type (ie. '1' for basic, '2' for advanced) | basic_id (nullable foreign key) | advanced_id (nullable foreign key) | email | name | address | phone (etc etc)
"users_basic":
id | user_id (foreign key) | (other data only required for basic users)
"users_advanced"
id | user_id (forgein key) | (other data only required for advanced users)
I get the feeling it's bad design cause there's no way to get all the data in one query without checking what type of user it is first, but I really don't like the idea of having ONE table with a ton of NULL data. What is the best way to design this?
Of course it is fine to have NULL foreign keys.
In your case, though, I'd be inclined to do one of two things. If there really aren't very many columns for the basic and advanced users, you can just include them in the users table. This would be the typical approach.
Otherwise, you can declare user_id as the primary key in all three tables, and still have a foreign key relationship from the secondary tables (users_basic and users_advanced) to the primary (users). Maintaining the distinctiveness of the relationship is tricky in MySQL and probably not worth doing.
I've created a database with three tables in it:
Restaurant
restaurant_id (autoincrement, PK)
Owner
owner_id (autoincrement, PK)
restaurant_id (FK to Restaurant)
Deal
deal_id (autoincrement)
owner_id (FK to Owner)
restaurant_id (FK to Restaurant)
(PK: deal_id, owner_id, restaurant_id)
There can be many owners for each restaurant. I chose two foreign keys for Deal so I can reference the deal by either the owner or the restaurant. The deal table would have three primary keys, two being foreign keys. And it would have two one-to-many relationships pointing to it. All of my foreign keys are primary keys and I don't know if I'll regret doing it like this later on down the road. Does this design make sense, and seem good for what I'm trying to achieve?
Edit: What I really need to be able to accomplish here is when a owner is logged in and viewing their account, I want them to be able to see and edit all the deals that are associated with that particular restaurant. And because there can be more that one owner per restaurant, I need to be able to perform a query something like: select *from deals where restaurant_id = restaurant_id. In other words, if I'm an owner and I'm logged in, I need to be able to make query: get all of the deal that are related to not just me, the owner, but to all of the owners associated with this restaurant.
You're having some trouble with terminology.
A table can only ever have a one primary key. It is not possible to create a table with two different primary keys. You can create a table with two different unique indexes (which are much like a primary key) but only one primary key can exist.
What you're asking about is whether you should have a composite or compound primary key; a primary key using more than one column.
Your design is okay, but as written you probably have no need for the column deal_id. It seems to me that restaurant_id and owner_id together are enough to uniquely identify a row in Deal. (This may not be true if one owner can have two different ownership stakes in a single restaurant as the result of recapitalization or buying out another owner, but you don't mention anything like that in your problem statement).
In this case, deal_id is largely wasted storage. There might be an argument to be made for using the deal_id column if you have many tables that have foreign keys pointing to Deal, or if you have instances in which you want to display to the user Deals for multiple restaurants and owners at the same time.
If one of those arguments sways you to adopt the deal_id column, then it, and only it, should be the primary key. There would be nothing added by including the other two columns since the autoincrement value itself would be unique.
If u have a unique field, this should be the PK, that would be the incremented field.
In this specific case it gives u nothing at all to add more fields to this key, it actually somewhat impacts performance (don't ask me how much, u bench it).
if you want to create 2 foreign keys in the deal table which are the restaurant and the owner the logic is something like a table could exist in the deal even without an owner or an owner could exist in the deal even without identifying the table on it but you could still identify the table because it's being used as a foreign key on the owner table, but if your going to put values on each columns that you defined as foreign key then I think it's going to be redundant cause I'm not sure how you would use the deal table later on but by it's name I think it speaks like it would be used to identify if a restaurant table is being reserved or not by a customer and to see how you have designed your database you could already identify the table which they have reserved even without specifying the table as foreign key in the deal table cause by the use of the owner table you would able to identify which table they have reserved already since you use it as foreign key on the owner table you just really have to be wise on defining relationships between your tables and avoid redundancy as much as possible. :)
I think it is not best.
First of all, the Deal table PK should be the deal_id. There is no reason to add additional columns to it--and if you did want to refer to the deal_id in another table, you'd have to include the restaurant_id and owner_id which is not good. Whether deal_id should also be the clustered index (a.k.a. index organized on this column) depends on the data access pattern. Will your database be full of data_id values most often used for lookup, or will you primarily be looking deals up by owner_id or restaurant_id?
Also, using two separate FKs way the you have described it (as far as I can tell!) would allow a deal to have an owner and restaurant combination that are not a valid (combining an owner that does not belong to that restaurant). In the Deal table, instead of one FK to Owner and one FK to Restaurant, if you must have both columns, there should be a composite FK to only the Owner table on (OwnerID, RestaurantID) with a corresponding unique key in the Owner table to allow this link up.
However, with such a simple table structure I don't really see the problem in leaving RestaurantID out of the Deal table, since the OwnerID always fully implies the RestaurantID. Obviously your deals cannot be linked only with the restaurant, because that would imply a 1:M relationship on Deal:Owner. The cost of searching based on Restaurant through the Owner table shouldn't really be that bad.
Its not wrong, it works. But, its not recommended.
Autoincrement Primary Keys works without Foreign Keys (or Master Keys)
In some databases, you cannot use several fields as a single primary key.
Compound Primary Keys or Compose Primary Keys are more difficult to handle in a query.
Compound Primary Key Query Example:
SELECT
D.*
FROM
Restaurant AS R,
Owner AS O,
Deal AS D
WHERE
(1=1) AND
(D.RestaurantKey = D.RestaurantKey) AND
(D.OwnerKey = D.OwnerKey)
Versus
Single Primary Key Query Example:
SELECT
D.*
FROM
Restaurant AS R,
Owner AS O,
Deal AS D
WHERE
(D.OwnerKey = O.OwnerKey)
Sometimes, you have to change the value of foreign key of a record, to another record. For Example, your customers already order, the deal record is registered, and they decide to change from one restaurant table to another. So, the data must be updated, in the "Owner", and "Deal" tables.
+-----------+-------------+
| OwnerKey | OwnerName |
+-----------+-------------+
| 1 | Anne Smith |
+-----------+-------------+
| 2 | John Connor |
+-----------+-------------+
| 3 | Mike Doe |
+-----------+-------------+
+-----------+-------------+-------------+
| OwnerKey | DealKey | Food |
+-----------+-------------+-------------+
| 1 | 1 | Hamburguer |
+-----------+-------------+-------------+
| 2 | 2 | Hot-Dog |
+-----------+-------------+-------------+
| 3 | 3 | Hamburguer |
+-----------+-------------+-------------+
| 1 | 3 | Soda |
+-----------+-------------+-------------+
| 2 | 1 | Apple Pie |
+-----------+-------------+-------------+
| 3 | 3 | Chips |
+-----------+-------------+-------------+
If you use compound primary keys, you have to create a new record for "Owner", and new records for "Deals", copy the other fields, and delete the previous records.
If you use single keys, you just have to change the foreign key of Table, without inserting or deleting new records.
Cheers.
i have two tables, one 'master' and one 'child' table.
Each table has a field named 'ProductNo', which is defined as PRIMARY KEY and UNIQUE.
Is it possible to define the field 'ProductNo' in the table 'child' and the same field in table 'master' as PRIMARY + UNIQUE together?
master:
ID | ProductNo
child:
ID | MasterID (FK on master.ID) | ProductNo
Relation >> 1 (master) : n (child)
example data:
master:
1 | 1234
2 | 4567
child:
100 | 1 | 3333
101 | 1 | 4444
102 | 2 | 5555
103 | 1 | 1234 <----- NOT ALLOWED! PRODUCT NO ALREADY EXISTING IN TABLE `MASTER`
104 | 2 | 1234 <----- NOT ALLOWED! PRODUCT NO ALREADY EXISTING IN TABLE `MASTER`
It is needed to check on inserting/updating table 'child' if 'ProductNo' already exists in table 'master'.
How can I define it?
Or am I needed to create a trigger for this?
TIA Matt
no, there is no such thing as composite PKs among tables.
Just for data consistency, if the Ids are the same, you should add a FK from child to the master.
To solve your problem, a trigger with a check like this:
if exists (select 1 from master where prodcutId=new_productId)
would be a good idea
EDIT:
actually the best idea is to have only one table called product with a ID and a masterID field with a relation to itself. The way you have today Im pretty sure that you have a lot of duplicate data and you are stuck with 2 levels on hierarchy.
(Original answer) You can declare a foreign key from master to child, even if that foreign key points to the primary key of child. This would be a one to zero-or-one relationship, and is not that uncommon. A row cannot exist in child without a matching row in master already being inserted, but a row can exist in master without a matching child row. Your inserts therefore need to happen in the order master then child.
(Edited in light of question edit) HOWEVER, in your case, the column you are referring to looks like it should not actually be the primary key of either table, but rather you have a separate primary/foreign key, and the column in question needs to be unique across the two tables, which has become clear now you've edited some sample data into your question. In this case, you'd be best to use a trigger on both tables, to check existence in the other table and prevent the insert/update if the ProductNo already exists.
Just as #DavidM said, it can be done, but it seems you are with some modelling issues. First, if you have a natural primary key ProductNo, why do you define a surrogate ID? The other thing you might consider is to combine these two tables into a single one (as might make sense for most of 1-to-1 cases).
Are you sure you need the two tables?
Keep just one, having productID plus parentID.
Then productID can be a primary key and auto increment, while everything having a parentID other than null (f.keyed to the same table) would be a child item.
You can add a column named ProductNo in child table and add a foreign key reference to the parent table.
Firstly, I apologise if this is a dupe - I suspect it may be but I can't find it.
Say I have a table of companies:
id | company_name
----+--------------
1 | Someone
2 | Someone else
...and a table of contacts:
id | company_id | contact_name | is_primary
----+------------+--------------+------------
1 | 1 | Tom | 1
2 | 2 | Dick | 1
3 | 1 | Harry | 0
4 | 1 | Bob | 0
Is it possible to set up the contacts table in such a way that it requires that one and only one record has the is_primary flag set for each common company_id?
So if I tried to do:
UPDATE contacts
SET is_primary = 1
WHERE id = 4
...the query would fail, because Tom (id = 1) is already flagged as the primary contact for company_id = 1. Or even better, would it be possible to construct a trigger so that the query would succeed, but Tom's is_primary flag would be cleared by the same operation?
I am not too bothered about checking whether company_id exists in the companies table, my PHP code would already have performed this check before I got to this stage (although if there is a way to do this in the same operation it would be nice, I suppose).
When I initially thought about this I thought "that will be easy, I'll just add a unique index across the company_id and is_primary columns" but obviously that won't work as it would restrict me to one primary and one non-primary contact - any attempt to add a third contact would fail. But I can't help feeling there would be a way to configure a unique index that gives me the minimum functionality I require - to reject an attempt to add a second primary contact, or reject an attempt to leave a company with no primary contact.
I am aware that I could just add a primary_contact field to the companies table with an FK to the contacts table but it feels messy. I don't like the idea of both tables having an FK to the other - it seems to me that the one table should rely on the other, not both tables relying on each other. I guess I just think that over time there is more chance of something going wrong.
To sum up:
How can I restrict the contacts table so that one and only one record with a given company_id has the is_primary flag set?
Anyone have any thoughts on whether two tables having FKs to each other is a good/bad idea?
Circular refenences between tables are indeed messy. See this (decade old) article: SQL By Design: The Circular Reference
The cleanest way to make such a constraint is to add another table:
Company_PrimaryContact
----------------------
company_id
contact_id
PRIMARY KEY (company_id)
FOREIGN KEY (company_id, contact_id)
REFERENCES Contact (company_id, id)
This will also require a UNIQUE constraint in table Contact on (company_id, id)
You could just do a query before that one setting
UPDATE contacts SET is_primary = 0 WHERE company_id = .....
or even
UPDATE contacts
SET is_primary = IF(id=[USERID],1,0)
WHERE company_id = (
SELECT company_id FROM contacts WHERE id = [USERID]
);
Just putting an alternative out there - personally I'd probably look to the FK approach though instead of this type of workaround i.e. have a field in the companies table with a primary_user_id field.
EDIT method w/o relying on a contact.is_primary field
Alternative method, first of all remove is_primary from contacts. Secondly add a "primary_contact_id" INT field into companies. Thirdly, when changing the primary contact, just change that primary_contact_id thus preventing any possibility of there being more than 1 primary contact at any time and all without the need for triggers etc in the background.
This option would work fine in any engine as it's simply updating an INT field, any reliance on FK's etc could be added/removed as required but at it's simplest it's just changing an INT fields value
This option is viable as long as you need one and precisely one link from companies to contacts flagging a primary
I've seen a lot of discussion regarding this. I'm just seeking for your suggestions regarding this. Basically, what I'm using is PHP and MySQL. I have a users table which goes:
users
------------------------------
uid(pk) | username | password
------------------------------
12 | user1 | hashedpw
------------------------------
and another table which stores updates by the user
updates
--------------------------------------------
uid | date | content
--------------------------------------------
12 | 2011-11-17 08:21:01 | updated profile
12 | 2011-11-17 11:42:01 | created group
--------------------------------------------
The user's profile page will show the 5 most recent updates of a user. The questions are:
For the updates table, would it be possible to set both uid and date as composite primary keys with uid referencing uid from users
OR would it be better to just create another column in updates which auto-increments and will be used as the primary key (while uid will be FK to uid in users)?
Your idea (under 1.) rests on the assumption that a user can never do two "updates" within one second. That is very poor design. You never know what functions you will implement in the future, but chances are that some day 1 click leads to 2 actions and therefore 2 lines in this table.
I say "updates" quoted because I see this more as a logging table. And who knows what you may want to log somewhere in the future.
As for unusual primary keys: don't do it, it almost always comes right back in your face and you have to do a lot of work to add a proper autoincremented key afterwards.
It depends on the requirement but a third possibility is that you could make the key (uid, date, content). You could still add a surrogate key as well but in that case you would presumably want to implement both keys - a composite and a surrogate - not just one. Don't make the mistake of thinking you have to make an either/or choice.
Whether it is useful to add the surrogate or not depends on how it's being used - don't add a surrogate unless or until you need it. In any case uid I would assume to be a foreign key referencing the users table.