Can you have a composite foreign key?
A composite foreign key is a foreign key that consists of two or more columns. It is important to note that all the columns in a single foreign key must point to the same table. In other words, it is not possible to have a foreign key that references to a column in Table 1 and a column in Table 2.
Can a composite foreign key be null?
A FOREIGN KEY constraint can contain null values; however, if any column of a composite FOREIGN KEY constraint contains null values, verification of all values that make up the FOREIGN KEY constraint is skipped.
Can a composite primary key be NULL in one column?
Primary key states that column mustn’t have NULL values. So columns used for defining composite primary key isn’t going to be NULL . Also Oracle server compares the combination of all columns used in a composite primary key definition.
What is an example of a composite key?
A composite key is a set of more than one key that, together, uniquely identifies each record. A foreign key, on the other hand, is a key in some table which uniquely identifies rows in another table – or, in other words, a key that tracks to a primary key in another table. An example would be a list of homes on a real estate market.
Does a foreign key have to be a primary key?
The foreign key or the columns in the referencing table must be the primary key or a candidate key (a key that can be used as the primary key) in the referenced table. Moreover, foreign keys allow linking data across several tables. Therefore, the foreign key cannot contain values that do not appear in the table…
Does a foreign key have to be unique?
By the SQL standard, a foreign key must reference either the primary key or a unique key of the parent table. If the primary key has multiple columns, the foreign key must have the same number and order of columns. Therefore the foreign key references a unique row in the parent table; there can be no duplicates.
Can unique key be used as foreign key?
Yes , There can be a foreign key which is unique key in other table as Unique key is subset of primary key but not the exact primary key. So that’s possible that foreign key is unique key in aother table. General standard answer is no. It is only possible if foreign key refers any column uniquely in other table.