Do foreign keys reference Primary Keys?
In a foreign key reference, a link is created between two tables when the column or columns that hold the primary key value for one table are referenced by the column or columns in another table. This column becomes a foreign key in the second table. For example, the Sales.
Can a foreign key reference two Primary Keys?
Rules for FK constraints To answer the question in the title and at the end of your text: “I would still like to know how to have one foreign key referencing two primary keys.” That’s impossible. A FOREIGN KEY constraint can only point to one table and each table can only have one PRIMARY KEY constraint.
Does foreign key have to be unique Oracle?
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 a table have two foreign keys and no primary key?
Yes, you can make one without a Primary Key (or, another option is a Compound Primary Key – making the two references a unique pair, and using that as the unique identifying key – but even this isn’t necessary (note: just because it “isn’t necessary” doesn’t mean it isn’t “good practice”; it wouldn’t generally be a …
What is primary key and unique key?
Primary key will not accept NULL values whereas Unique key can accept NULL values. A table can have only one primary key whereas there can be multiple unique key on a table. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.
Does foreign key have to be unique?
Although primary key values must be unique, foreign key values are not required to be unique. For example, a single customer at a bank might have multiple accounts.
How is a foreign key different from a primary key?
The primary key doesn’t allow null values. Foreign key accepts multiple null values. It can identify the record uniquely in the database table. A foreign key is a field in the table that is the primary key in another table.
Can a table have two foreign keys from the same table?
FOREIGN KEY constraints can reference another column in the same table, and is referred to as a self-reference. A FOREIGN KEY constraint specified at the column level can list only one reference column. This column must have the same data type as the column on which the constraint is defined.
Is foreign key unique in a table?
Foreign keys. A foreign key is a column (or group of columns) within a table that refers to (or “relates to”) a unique value in a referenced table. Each value in the foreign key column must have a matching value in the referenced table.
Can foreign key refer to unique?
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.
Can you have 2 foreign keys in a table?
A table may have multiple foreign keys, and each foreign key can have a different parent table. Each foreign key is enforced independently by the database system. Therefore, cascading relationships between tables can be established using foreign keys.
Can a table only have foreign key without primary key?
You need to have either a primary key or unique key in the column which will be referred by the foreign key. Without it you can’t create the foreign key.