Is it mandatory for a foreign key to be a primary key in another table?
A foreign key is a column or a set of columns in a table whose values are required to match at least one primary key or unique key value of a row in its parent table.
Does a foreign key have to be a primary key in another table?
A foreign key can refer to either a unique or a primary key of the parent table. If the foreign key refers to a non-primary unique key, you must specify the column names of the key explicitly.
Does a foreign key need to have the same name as its referenced primary key?
Since each foreign key value must exactly match the corresponding primary key value, the foreign key must contain the same number and data type of columns as the primary key, and these key columns must be in the same order. A foreign key can also have different column names than the primary key.
Can the candidate key be a foreign key of another table?
A candidate key's versatility allows it to serve as a foreign key in another table, providing relational database designers with flexibility while ensuring the establishment and maintenance of meaningful relationships between tables.
In SQL you can perform JOIN s whether or not there's a foreign-key constraint. All that matters is that the data types match up (in this case integers).
Why does each foreign key have to reference a primary key?
Foreign keys are vital in maintaining data integrity across tables. The reference to a valid primary key in another table helps prevent orphaned records (records with foreign fundamental values that don't correspond to any existing data in the referenced table).
A foreign key is one or more columns in a table that references the primary key of another, creating a link between them. Foreign keys cannot exist without being linked to a primary key.
Is foreign key mandatory for the creation of primary key?
In SQL, yes, the target reference of a foreign key constraint must normally be a primary key, though in some SQL dialects it is sufficient for the foreign key constraint to reference a column or columns with a UNIQUE constraint.
The foreign key column in the child table should have the same data type as the primary key in the parent table. Both tables should be present in the same database. The referenced table should contain unique values.
Foreign keys are important for several reasons, including the following: Streamline data sets. With foreign keys, database administrators don't have to store repeated data in multiple tables. They make data available to different tables without creating redundant data sets.
However you can't add many foreign key constraints to one field, because that is not allowed in mysql. A key can't point to multiple tables! So polymorphic relationships fix your issue, but you still won't have foreign keys!
There is no problem having a table that consists of foreign keys only. In your particular example it would be OK to remove the OrderID. Relational theory states that each table should have a candidate key so, ideally, the business rule you mention would be in place to make the foreign key only table a "good" table.
Can a table have only one primary key but multiple foreign keys?
A primary key uniquely identifies a row In the table. A foreign key references the primary (or in some systems any unique key) in another table thereby establishing a relationship between the two tables. A table may have only one primary key, but may have zero to many other unique keys.
Can we have primary key and foreign key in same table?
Yes. It is possible. Example: an employee table with emp_id as primary key might also include a manager column as foreign key to the emp_id of the same table.
The short answer is YES. A database table can exist without a primary key. But without having a primary key on a table you are leaving the door open for potential data management/inconsistency headaches.
Can a key be called a foreign key if it is not a primary key?
A foreign key is one or more columns that refer to a primary key. The data types of the foreign and primary keys must be the same, but the names may be different. Foreign keys have different rules than primary keys: Foreign key values may be repeated. Foreign key values must match up to a primary key value.
Does a foreign key have to be a primary key in Postgres?
A foreign key must reference columns that either are a primary key or form a unique constraint, or are columns from a non-partial unique index. This means that the referenced columns always have an index to allow efficient lookups on whether a referencing row has a match.
The obvious problem with the lack of foreign keys is that a database can't enforce referential integrity and if it wasn't taken care of properly at the higher level then this might lead to inconsistent data (child rows without corresponding parent rows).
Foreign keys are just a constraint on the data that can be entered into the table, it is not a prerequisite for joining tables. You can join on whatever you choose.
Should foreign key and primary key have the same name?
1) Name of foreign key can be different than the name of primary key it represent in other table. For example in our Employee and Department relationship, Primary key in Department table is dept_id and we have used same name in Employee table to create foreign key.
