I want to create a many-to-one relationship between my customer and booking tables, however when I enforce referential integrity, only one-to-one shows.
I have several other tables that have worked correctly, so this seems to be a bug; can anyone offer any suggestions?
Summary:
Customer ID is the unique primary key in the Customer table
Customer ID is a foreign key in the booking table
Access is enforcing a one-one relationship between the two
How can I create this many-to-one relationship?
Thank you to everyone who responded, it turned out I had accidentally set the field indexing property to NO Duplicates.
Related
I want to make an ER diagram involving a 1 to many relationship. Should I add the attribute of the relationship on the N side too? And if possible write the correct ER diagram.
Take a simple example of many people working in one department with a constraint that a single person can work only in one department. That makes department connected to people in a one-to-many fashion.
The simplest way to represent this is to add the primary key of you department table as a foreign key in your people table.
The schema will look something like this
DEPARTMENT (dept_no, dept_name, dept_location,....)
PEOPLE (p_no, dept_works, p_name, p_dob,....)
Where, dept_works in PEOPLE table is the foreign key that references the dept_no Primary key in the DEPARTMENTS table. This way you can store the ID of the department for each person in which he/she works.
I have two entities - Request and Report. Each Request can have only one Report and each Report can belong to only one Request. This is one-to-one relationship.
Right now I'm thinking about underlying database schema. Right now in table requests I have FK to PK reports.id.
Is it a correct approach ? Where FK to other entity must be placed - in reports or requests table ?
That's all you need regarding the foreign keys. You may want to also add a unique constraint (unique index) to the foreign key for better performance and consistency guarantees.
When having a one-to-one relationship in a database the other table has a foreign key ID (in this example). And in a one-to-many relationship the table contains many foreign keys.
But does the database know whether this is a one-to-one or one-to-many relationship? Are the relationships that I make in an ER-Diagram only to indicate where there should be foreign keys when making the actual tables?
What is the difference between one-to-one and one-to-many relationship in a database?
In a sense, all the relationships we talk about are not known to the database, they are constructs we have invented to better understand how to design the tables.
The big difference in terms of table structure between one-to-one and one-to-many is that in one-to-one it is possible (but not necessary) to have a bidirectional relationship, meaning table A can have a foreign key into table B, and table B can have a foreign key into the associated record in table A. This is not possible with a one-to-many relationship.
One-to-one relationships associate one record in one table with a single record in the other table. One-to-many relationships associate one record in one table with many records in the other table.
To enable one-to-one relationship you need to add unique constraint to foreign key. It is not possible to have two foreign keys for each table as it will be impossible to create records.
Im having trouble understanding what the actual question is.
Your analysis is for the most part correct, in that if you have a 2 tables, and table2 has a foreign key to table one, it could be either a one-to-one or a many-to-one.
Your sentence "And in a one-to-many relationship the table contains many foreign keys."
The table of the 'many' side still contains one column that is a foreign key, its just that more than one row can have the same foreign key value (many rows point to one parent).
Also note that you can put the foreign key on the parent table, to the child, instead of the other way around. In this way, you can prevent one-to-many if you want to do that. Also note that in this way, more than one parent can share a child, which might or might not be what you want.
The database-level equivalent of a 1:1 vs. 1:m is having a unique index on the foreign key column. Note that this will only work for 1:1, NOT 1:0..1, as null is considered when evaluating uniqueness. There are workarounds for this restriction, but that's it at the basic level.
Similarly by example, a product has only one product code, so it's one-to-one relationship (product <-> ABC123), but a customer can purchase more than one product, so it's one-to-many relationship (person <->>>product).
well, you are right, this relation is important for you, but not for db itself. When you have two tables, one with your basic information, and another one with your detailed information.. for both tables you are you, so it is one-to-one relation, you can not map your data to somebody else.
Now add third table "cities" and one of your information points to city you live in - this is example of one-to-many (one city can be used, and should be used for many people).
one-to-many / one-to-one just show how your tables interact. And all the time, you want to "save" rows/columns in table not duplicating them you will use one-to-many relation with another table. Or many-to-many :)
Let's assume you have a table with two attributes A and B. If A is a candidate key and B is not then the relationship between A and B is 1 to many. If both A and B are candidate keys then the relationship is 1 to 1.
Given table A and B if
A and B have a strict 1 to 1 relationship
For every B instance, there will always be an A instance
The best approach is to make the primary key of B also a foreign key referencing A. This is also called "Table per Type Inheritance" and the "is a" relationship. There are other ways to enforce a unique foreign key, but using the primary key makes the relationship clear in the schema and in ER diagrams.
Of course there are always other scenarios, and if your design doesn't meet both of the criteria above, you'll have to use another approach.
This is a basic database design question. I want a table (or multiple tables) defining relationships between customers. I want it so PrimaryCustomer can be linked to multiple SecondaryCustomers, and can have many SecondaryCustomers with the same relationship.
PrimaryCustomerID RelationshipID SecondaryCustomerID
1) If the primary key is {PrimaryCustomerID} then I can only have one linked customer of any kind.
2) If the primary key is {PrimaryCustomerID, RelationshipID}, then I can only have one linked customer for each relationship type.
3) If the primary key is {PrimaryCustomerID, RelationshipID, SecondaryCustomerID}, then I can have whatever I like, but having all columns as the primary key seems completely wrong.
What's the right way to set things up?
A third alternative might be for the key to be (PrimaryCustomerId, SecondaryCustomerId), which would make sense if only one type of relationship is permitted per pair of customers. What keys to implement should be defined by what dependencies you need to represent in the table so that the table accurately represents the reality you are modelling. There's nothing wrong in principle with compound keys or all-key tables.
Number 3 is the right way to go for this data model. Linking tables often have all the columns in a join as all they do is link to other tables.
If a customer can only be linked to one primary customer then you can use a simple recursive relationship in the customer table itself.
CustomerID as PK
PrimaryCustomerID as FK to CustomerID
Nothing wrong with No 3.
If you need to prevent reverse-relationship duplicates, you can use
ALTER TABLE CustomerRelationship
ADD CONSTRAINT chk_id CHECK (PrimaryCustomerId < SecondaryCustomerId);
If I want to implement the relationship between Category and Classified, is a database-level nullable foreign key required or is it possible/advisable for an application to define this type of relationship without using a database constraint?
[Note: Because the white dot indicates "optional" and the black dot "required", for each Category a Corresponding classified may or may not exist. In addition, the crows feet between them indicate this is a many to many relationship.]
Since it's a many-to-many relationship, you'll want a cross-reference table rather than a simple foreign key column.
So the Category table does not have a FK to Classified, and Classified does not have a FK to Category. Instead you can have a new table :
XrefCategoryClassified
FK to Category NOT NULL
FK to Classified NOT NULL
This is a typical way to implement a many-to-many relationship. And now, instead of worrying about NULLable fields if two records aren't related, you simply care about the existence or non-existence of a xref record
Why not use both?
Foreign keys, check constraints etc are known as "Declarative Referential Integrity" for a reason. They protect your data. What if you add a bulk load next month, or you have to run a SQL script to change data?
Another point would be that the database engine is the correct tool for this.
Absent compelling reasons to do otherwise, I'd enforce referential integrity at the database level; after all that's (partly) what an RDBS is good for.
And since you'll likely have some sort of mapping table to define the many-to-many relationship between Category and Classified, it seems like a no-brainer to put your constraints there. Your queries will thank you for it later.