A foreign key can refer to a specific table or to the entire database. For example, if you’re using SQL Server, it’s best to define a foreign key so it refers to the table that the primary key is set on. If you’re using Oracle, the foreign key can be defined in the database and it may be a single column in the table.
A foreign key in SQL Server refers only to a table and is not necessarily a column. In other databases, a foreign key can refer to the entire table, such as in Oracle. For example, in SQL Server, if you defined a foreign key to the entire table, with a foreign key, that would mean you have to define a foreign key to every column in the table.
This is why I think you are going to want to use a Foreign Key in SQL Server. That way we can make sure that any column that is linked to any other column will have an identity column. A foreign key in SQL Server does this by linking columns together by referencing every row in every table, whether or not that row has a matching foreign key. This is similar to a JOIN in SQL Server.
Since columns that are foreign keys can be linked to any other column, it’s a good idea to ensure that any foreign key columns are defined with a foreign key that is also defined in the primary key. It will prevent column conflicts. If you don’t, then one of your columns will be auto-incrementing, and it’s difficult to determine a row’s id. The foreign key will help prevent your queries from getting a “Concurrency violation” error.
SQL Server is a good example of a DBMS that is often difficult to understand. Many DBMS’s do not have a foreign key relationship to the primary key, or there is a foreign key that is not defined. So I would suggest to the reader that they consider using a different database, like MySQL.
MySQL is a great database because it is well understood, has a great documentation, and is easy to use. So many people that have used MySQL claim that they don’t need foreign keys. I disagree. If you are not using foreign keys, you need to be able to define this relationship so that you can use the foreign key. The foreign key is just the key that references the primary key.
Yes, I agree that we should consider using another database. But there are many reasons why I think that a foreign key should be defined in one table and NOT in another. I’ll discuss these in depth later on.
The thing with foreign keys comes down to how many tables they reference. If you have a one-to-many relationship, then you will need to have a foreign key. But if you have a many-to-many relationship, then you dont need a foreign key. The first table has to reference the second. If not, you can create a different table that references the two. If you have a one-to-one relationship, then you can use both.
With a one-to-one relationship, you would only require a single foreign key. But with a many-to-many relationship, you need to go beyond just a single foreign key. You will need to have a foreign key in one table, and a foreign key in another table. The problem with this is that for some relationships, you can’t just put the foreign key in one table and have it reference another table that references the two.
This is easily solved with the foreign key reference in both tables. For example, if you have a many-to-many relationship between Person and Address, you would only need a single foreign key reference in the Address table. But if you have a many-to-one relationship, you will need to go beyond a single foreign key. You will need to have a foreign key in one table, and a foreign key in the other.