Suppose I have a table that contains a list of ids pointing to table names, with the following schema:
RemoteTables
============
id (Primary Key)
tableName (string)
Suppose I also have the following schema in another table:
AnotherTable
============
id (Primary Key)
remoteTableId (foreign key referencing RemoteTables)
remoteId ("special" foreign key)
where remoteId points to a row in the table specified by remoteTableId.
I have seen a few situations where I wanted to do this, but is there any built-in SQL support to manage the relationship in any way?
What you have is a special type of constraint here, because you are building a custom meta-database (data dictionary of sorts). So nothing "built in", per se.
Your best bet would be to enforce this through a trigger, although triggers on MySQL unfortunately are not always as easy as they ought to be.
Related
TL;DR
Should a foreign key always refer to the id column of another table? Why or why not? Is there a standard rule for this?
Is there a cost associated with using any other unique column other than id column for foreign key? Performance / storage? How significant? Is it frowned in the industry?
Example: this is the schema for my sample problem:
In my schema sometimes I use id column as the foreign key and sometimes use some other data column.
In vehicle_detail table I use a unique size column as a foreign key from vehicle_size table and unique color column as the foreign key from vehicle_color table.
But in vehicle_user I used the user_identifier_id as a foreign key which refers to id primary key column in user_identifier table.
Which is the correct way?
On a side note, I don't have id columns for the garage_level, garage_spaceid, vehicle_garage_status and vehicle_parking_status tables because they only have one column which is the primary key and the data they store is just at most 15 rows in each table and it is probably never going to change. Should I still have an id column in those ?
A foreign key has to target a primary key or unique constraint. It is normal to reference the primary key, because you typically want to reference an individual row in another table, and the primary key is the identifier of a table row.
From a technical point of view, it does not matter whether a foreign key references the primary key or another unique constraint, because in PostgreSQL both are implemented in the same way, using a unique index.
As to your concrete examples, there is nothing wrong with having the unique size column of vehicle_size be the target of a foreign key, although it begs the question why you didn't make size the primary key and omit the id column altogether. There is no need for each table to have an id column that is the automatically generated numeric primary key, except that there may be ORMs and other software that expect that.
A foreign key is basically a column of a different table(it is always of a different table, since that is the role it serves). It is used to join/ get data from a different table. Think of it like say school is a database and there are many different table for different aspects of student.
say by using Admission number 1234, from accounts table you can get the fees and sports table you can get the sports he play.
Now there is no rule that foreign key should be id column, you can keep it whatever you want. But,to use foreign key you should have a matching column in both tables therefore usually id column is only used. As I stated in the above example the only common thing in say sports table and accounts table would be admission number.
admn_no | sports |
+---------+------------+
| 1234 | basketball
+---------+---------+
| admn_no | fees |
+---------+---------+
| 1234 | 1000000 |
+---------+---------+
Now say using the query\
select * from accounts join sports using (admn_no);
you will get:
+---------+---------+------------+
| admn_no | fees | sports |
+---------+---------+------------+
| 1234 | 1000000 | basketball |
+---------+---------+------------+
PS: sorry for bad formatting
A foreign key is a field or a column that is used to establish a link between two tables. A FOREIGN KEY is a column (or collection of columns) in one table, that refers to the PRIMARY KEY in another table.
There is no rule that it should refer to a id column but the column it refers to should be the primary key. In real scenarios, it usually refers to Id column as in most cases it is the primary key in the tables.
OP question is about "correct way".
I will try to provide some kind of summary from existing comments and answers, general DO and general DONT for FKs.
What was already said
A. "A foreign key has to target a primary key or unique constraint"
Literally from Laurenz Albe answer and it was noted in comments
B. "stick with whatever you think will change the least"
It was noted by Adrian Klavier in comments.
Notes
There is no such general rule that PK or unique constraint must be defined on a single column.
So the question title itself must be corrected: "Which column(s) for foreign key: id or any other column(s) and why?"
Let's talk about "why".
Why: General DO, general DONT and an advice
Is there a cost associated with using any other unique column other than id column for foreign key? Performance / storage? How significant? Is it frowned in the industry?
General DO: Analyze requirements, use logic, use math (arithmetics is enough usually). There is no a single database design that's always good for all cases. Always ask yourself: "Can it be improved?". Never be content with design of existing FKs, if requirements changed or DBMS changed or storage options changed - revise design.
General DONT: Don't think that there is a single correct rule for all cases. Don't think: "if that worked in that database/table than it will work for this case too".
Let me illustrate this points with a common example.
Example: PK on id uuid field
We look into database and see a table has a unique constraint on two fields of types integer (4 bytes) + date (4 bytes)
Additionally: this table has a field id of uuid type (16 bytes)
PK is defined on id
All FKs from other tables are targeting id field
It this a correct design or not?
Case A. Common case - not OK
Let's use math:
unique constraint on int+date: it's 4+4=8 bytes
data is never changed
so it's a good candidate for primary key in this table
and nothing prevents to use it for foreign keys in related tables
So it looks like additional 16 bytes per each row + indexes costs is a mistake.
And that's a very common mistake especially in combination of MSSQL + CLUSTERED indexes on random uuids
Is it always a mistake?
No.
Consider latter cases.
Case B. Distributed system - OK
Suppose that you have a distributed system:
ServerA, ServerB, ServerC are sources of data
HeadServer - is data aggregator
data on serverA-ServerC could be duplicated: the same record could exists on several instances
aggregated data must not have duplicates
data for related tables can come from different instances: data for table with PK from serverA and data for tables with FKs from serverB-serverC
you need to log from where each record is originated
In such case existence of PK on id uuid is justified:
unique constraint allows to deduplicate records
surrogate key allows related data come from different sources
Case C. 'id' is used to expose data through API - OK
Suppose that you have an API to access data for external consumers.
There is a good unique constraint on:
client_id: incrementing integer in range 1..100000
invoice_date: dates '20100101'..'20210901'
And a surrogate key on id with random uuids.
You can create external API in forms:
/server/invoice/{client_id}/{invoice_date}
/server/invoice/{id}
From security POV /{id} is superior by reasons:
it's impossible to deduce from one uuid value existence of other
it's easier to implement authorization system for entities of different types. E.g. entityA has natural key on int, entityB on bigint' and entityC on int+ byte+date`
In such case surrogate key not only justified but becames essential.
Afterword
I hope that I was clear in explanation of main correct principle: "There is no such thing as a universal correct principle".
An additional advice: avoid CASCADE UPDATE/DELETEs:
Although it depends on DBMS you use.
But in general :
"explicit is better than implicit"
CASCADEs rarely works as intended
when CASCADES works - usually they have performance problems
Thank you for your attention.
I hope this helps somebody.
This may seem like a simple question, but I am stumped:
I have created a database about cars (in Oracle SQL developer). I have amongst other tables a table called: Manufacturer and a table called Parentcompany.
Since some manufacturers are owned by bigger corporations, I will also show them in my database.
The parentcompany table is the "parent table" and the Manufacturer table the "child table".
for both I have created columns, each having their own Primary Key.
For some reason, when I inserted the values for my columns, I was able to use the same value for the primary key of Manufacturer and Parentcompany
The column: ManufacturerID is primary Key of Manufacturer. The value for this is: 'MBE'
The column: ParentcompanyID is primary key of Parentcompany. The value for this is 'MBE'
Both have the same value. Do I have a problem with the thinking logic?
Or do I just not understand how primary keys work?
Does a primary key only need to be unique in a table, and not the database?
I would appreciate it if someone shed light on the situation.
A primary key is unique for each table.
Have a look at this tutorial: SQL - Primary key
A primary key is a field in a table which uniquely identifies each
row/record in a database table. Primary keys must contain unique
values. A primary key column cannot have NULL values.
A table can have only one primary key, which may consist of single or
multiple fields. When multiple fields are used as a primary key, they
are called a composite key.
If a table has a primary key defined on any field(s), then you cannot
have two records having the same value of that field(s).
Primary key is table-unique. You can use same value of PI for every separate table in DB. Actually that often happens as PI often incremental number representing ID of a row: 1,2,3,4...
For your case more common implementation would be to have hierarchical table called Company, which would have fields: company_name and parent_company_name. In case company has a parent, in field parent_company_name it would have some value from field company_name.
There are several reasons why the same value in two different PKs might work out with no problems. In your case, it seems to flow naturally from the semantics of the data.
A row in the Manufacturers table and a row in the ParentCompany table both appear to refer to the same thing, namely a company. In that case, giving a company the same id in both tables is not only possible, but actually useful. It represents a 1 to 1 correspondence between manufacturers and parent companies without adding extra columns to serve as FKs.
Thanks for the quick answers!
I think I know what to do now. I will create a general company table, in which all companies will be stored. Then I will create, as I go along specific company tables like Manufacturer and parent company that reference a certain company in the company table.
To clarify, the only column I would put into the sub-company tables is a column with a foreign key referencing a column of the company table, yes?
For the primary key, I was just confused, because I hear so much about the key needing to be unique, and can't have the same value as another. So then this condition only goes for tables, not the whole database. Thanks for the clarification!
I got this composite primary key in Table 1:
Table 1: Applicant
CreationDate PK
FamilyId PK
MemberId PK
I need to create a foreign key in Table 2 to reference this composite key. But i do not want to create three fields in Table 2 but to concatenate them in a single field.
Table 2: Sales
SalesId int,
ApplicantId -- This should be "CreationDate-FamilyId-MemberId"
What are the possible ways to achieve this ?
Note: I know i can create another field in Table 1 with the three columns concatenation but then i will have redundant info
What you're asking for is tantamount to saying "I want to treat three pieces of information as one piece of information without explicitly making it one piece of information". Which is to say that it's not possible.
That said, there are ways to make happen what you want to happen
Create a surrogate key (i.e. identity column) and use that as the FK reference
Create a computed column that is the concatenation of the three columns and use that as the FK reference
All else being equal (ease of implementation, politics, etc), I'd prefer the first. What you have is really a natural key and doesn't make a good PK if it's going to be referenced externally. Which isn't to say that you can't enforce uniqueness with a unique key; you can and should.
I have a table 'person' having a attribute ID (primary key). I want to store an array of person IDs in another table. How to define the foreign key in that table referring to table 'person'?
In the creation or alteration of your table include: Foreign_key_name int FOREIGN KEY REFERENCES Person(ID), assuming that Person.ID is of type int.
The exact command syntax might differ depending on your database type.
A good guide giving examples for MySQL / SQL Server / Oracle / MS Access can be found here
I need to make some changes to a SQL Server 2008 database.
This requires the creation of a new table, and inserting a foreign key in the new table that references the Primary key of an already existing table. So I want to set up a relationship between my new tblTwo, which references the primary key of tblOne.
However when I tried to do this (through SQL Server Management Studio) I got the following error:
The columns in table 'tblOne' do not
match an existing primary key or
UNIQUE constraint
I'm not really sure what this means, and I was wondering if there was any way around it?
It means that the primary key in tblOne hasn't been properly declared - you need to go to tblOne and add the PRIMARY KEY constraint back onto it.
If you're sure that tblOne does have a PRIMARY KEY constraint, then maybe there are multiple tblOne tables in your DB, belonging to different schemas, and your references clause in your FK constraint is picking the wrong one.
If there's a composite key (which your comment would indicate), then you have to include both columns in your foreign key reference also. Note that a table can't have multiple primary keys - but if it has a composite key, you'll see a key symbol next to each column that is part of the primary key.
If you have a composite key the order is important when creating a FK, and sometimes the order is not how it is displayed.
What I do is go to the Keys section of the table1 and select script primary key as create to clipboard and then create FK using the order as shown in script
I've had this situation that led me to this topic. Same error but another cause. Maybe it will help someone.
Table1
ColA (PK)
ColB (PK)
ColC
Table2
ID (PK)
ColA
COLB
When trying to create foreign key in Table2 I've choose values from combobox in reverse order
Table1.ColB = Table2.ColB
Table1.ColA = Table2.ColA
This was throwing me an error like in topic name. Creating FK keeping order of columns in Primary key table as they are, made error disappear.
Stupid, but.. :)
If you still get that error after you have followed all advice from the above answers and everything looks right.
One way to fix it is by Removing your Primary keys for both tables, Save, Refresh, and add them again.
Then try to add your relationship again.
This Error happened with me When I tried to add foreign key constraint starting from PrimaryKey Table
Simpy go to other table and and create this foreign key constraint from there (foreign key Table)
This issue caught me out, I was adding the relationship on the wrong table. So if you're trying to add a relationship in table A to table B, try adding the relationship in table B to table A.
That looks like you are trying to create a foreign key in tblTwo that does not match (or participate) with any primary key or unique index in tblOne.
Check this link on MSDN regarding it. Here you have another link with a practical case.
EDIT:
Answwering to your comment, I understand you mean there are 2 fields in the primary key (which makes it a composite). In SQL it is not possible to have 2 primary keys on the same table.
IMHO, a foreign key field should always refer to a single register in the referenced table (i.e. the whole primary key in your case). That means you need to put both fields of the tblOne primary key in tblTwo before creating the foreign key.
Anyway, I have investigated a bit over the Internet and it seems SQL Server 2008 (as some prior versions and other RDBMS) gives you the possibility to reference only part of the primary key as long as this part is a candidate key (Not Null and Unique) and you create an unique constraint on it.
I am not sure you can use that in your case, but check this link for more information on it.
I have found that the column names must match.
Example:
So if tblOne has id called categoryId a reference in tblTwo must also be called categoryId.
_tblname, primary key name, foreign key_
tblOne, "categoryId", none
tblTwo, "exampleId", "categoryId"
I noticed this when trying to create foreign key between 2 tables that both had the column name "id" as primary key.
If nothing helps, then this could be the reason:
Considering this case:
Table A:
Column 1 (Primary Key)
Column 2 (Primary Key)
Column 3
Column 4
Table B:
Column a (Primary Key)
Column b
Column c
when you are defining a dependency B to A, then you are forced to respect the order in which the primaries are defined.
That's mean your dependency should look like this:
Table A Table B
Column 1 Column b
Column 2 Column c
AND NOT:
Table A Table B
Column 2 Column c
Column 1 Column b
then this will lead to the error you are encountering.
I've found another way to get this error. This can also happen if you are trying to make a recursive foreign key (a foreign key to the primary key in the same table) in design view in SQL Management Studio. If you haven't yet saved the table with the primary key it will return this message. Simply save the table then it will allow you to create the foreign key.
If you have data in your tables this could be the issue.
In my case I had some data in the Account table that I loaded at 3 pm, and some data in Contact table that I loaded at 3:10 pm, so Contact table had some values that weren't in my Account table yet.
I ended up deleting these values from the contact table and then managed to add a key without any problems.
Kindly also see that there are no existing data inside the table where the primary key is defined while setting the foreign key with another table column.
this was the cause of the error in my case.
I had to take backup empty the table set the relationship and then upload the data back.
sharing my experience
Was using ms sql smss