I have encountered a situation while developing a star schema. I have a table like this
Name
Email
amy
amy#gmail.com
jess
amy#gmail.com
I want to find the key column as foreign key for Fact table as you can see there is a duplication of records if look individually but unique if consider both column as a key column
Your help will be highly regarded
I am assuming from your question that the table is a dimension? If that is the case then it should have a surrogate key as its PK (as every dimension table should) and you use this to join to fact tables.
I want to define a foreign key constraint between the table Speler and the table Wedstrijd. I want the key on the Wedstrijd table, but when I use this code in my SQL console:
ALTER TABLE Speler
ADD FOREIGN KEY (idSpeler) REFERENCES Wedstrijd(idWedstrijd);
It puts a key on the table Speler and on the table Wedstrijd
Thanks for your time!
ALTER TABLE Wedstrijd
ADD FOREIGN KEY (idWedstrijd) REFERENCES Speler(idSpeler);
Instead of "solving" what you perceive as your problem, I think you have a different problem:
By mapping idSpeler to idWedstrijd, you are basically saying that a Speler (Player) is equal to a Wedstrijd (Match). That becomes a 1:1 relation which is then shown as a line with two yellow 'key'-endings (assuming you are using SQL Server).
It is very likely to me that instead you need to create a linking table WedstrijdSpeler that sits between the other two tables.
Then the new table WedstrijdSpeler needs to be given 2 Foreign Keys:
WedstrijdSpeler.idWedstrijd -> Wedstrijd.idWedstrijd
WedstrijdSpeler.idSpeler -> Speler.idSpeler.
Then you can give WedstrijdSpeler either a combined Primary Key (containing both fields idWedstrijd and idSpeler), or you can add a third field idWedstrijdSpeler and make that the Primary Key. Either approach will do, it is up to you.
Very new to SQL and have spent a day on this already.
Here are my two tables:
Centre(cid, name, location, nurse_supervisor)
Nurse(nid, name, centre_id, certificate)
I have a big problem. The (nurse_supervisor) in Centre is a foreign key to Nurse (nid).
The (centre_id) in Nurse is a foreign key to (Centre cid).
I can't figure out how to populate these tables. I have tried:
INSERT ALL, which produces "A foreign key value has no matching primary key value"
I have tried removing the foreign key constraints and adding them after populating the tables but when I do that it says I can't add a constraint to tables with preexisting data.
I tried removing NOT NULL - but realized that was silly as the constraints will be enforced anyways.
Everything I look through says populate the parent table first and then the child, but these tables are linked to each other.
I am using SQL developer.
This is a poor schema design, but one way to get around it would be to:
Make both centre_id and nurse_supervisor columns NULL in the two table definitions
Insert all rows into both tables, but with NULL for those two columns
Update centre_id to the correct value for each row in the Nurse table
Update nurse_supervisor to the correct value for each row in the Centre table
I'm using Microsoft SQL Server Management Studio and while creating a junction table should I create an ID column for the junction table, if so should I also make it the primary key and identity column? Or just keep 2 columns for the tables I'm joining in the many-to-many relation?
For example if this would be the many-to many tables:
MOVIE
Movie_ID
Name
etc...
CATEGORY
Category_ID
Name
etc...
Should I make the junction table:
MOVIE_CATEGORY_JUNCTION
Movie_ID
Category_ID
Movie_Category_Junction_ID
[and make the Movie_Category_Junction_ID my Primary Key and use it as the Identity Column] ?
Or:
MOVIE_CATEGORY_JUNCTION
Movie_ID
Category_ID
[and just leave it at that with no primary key or identity table] ?
I would use the second junction table:
MOVIE_CATEGORY_JUNCTION
Movie_ID
Category_ID
The primary key would be the combination of both columns. You would also have a foreign key from each column to the Movie and Category table.
The junction table would look similar to this:
create table movie_category_junction
(
movie_id int,
category_id int,
CONSTRAINT movie_cat_pk PRIMARY KEY (movie_id, category_id),
CONSTRAINT FK_movie
FOREIGN KEY (movie_id) REFERENCES movie (movie_id),
CONSTRAINT FK_category
FOREIGN KEY (category_id) REFERENCES category (category_id)
);
See SQL Fiddle with Demo.
Using these two fields as the PRIMARY KEY will prevent duplicate movie/category combinations from being added to the table.
There are different schools of thought on this. One school prefers including a primary key and naming the linking table something more significant than just the two tables it is linking. The reasoning is that although the table may start out seeming like just a linking table, it may become its own table with significant data.
An example is a many-to-many between magazines and subscribers. Really that link is a subscription with its own attributes, like expiration date, payment status, etc.
However, I think sometimes a linking table is just a linking table. The many to many relationship with categories is a good example of this.
So in this case, a separate one field primary key is not necessary. You could have a auto-assign key, which wouldn't hurt anything, and would make deleting specific records easier. It might be good as a general practice, so if the table later develops into a significant table with its own significant data (as subscriptions) it will already have an auto-assign primary key.
You can put a unique index on the two fields to avoid duplicates. This will even prevent duplicates if you have a separate auto-assign key. You could use both fields as your primary key (which is also a unique index).
So, the one school of thought can stick with integer auto-assign primary keys, and avoids compound primary keys. This is not the only way to do it, and maybe not the best, but it won't lead you wrong, into a problem where you really regret it.
But, for something like what you are doing, you will probably be fine with just the two fields. I'd still recommend either making the two fields a compound primary key, or at least putting a unique index on the two fields.
I would go with the 2nd junction table. But make those two fields as Primary key. That will restrict duplicate entries.
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