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Should foreign keys be nullable?
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table).
Can a nullable column be a foreign key?
The number of columns in the foreign key must be equal to the number of columns in the corresponding primary or unique constraint (called a parent key) of the parent table. A table can have many foreign keys. A foreign key is nullable if any part is nullable. A foreign key value is null if any part is null.
What is nullable column?
For writable tables, any column that is going to store an SQL NULL value must have a null condition-name. If you have a table with multiple record types, it is possible to mark columns as nullable without an underlying null condition-name. This allows the column PRODUCTSIZE to be nullable. …
Can a foreign key be null and/or duplicate?
By default there are no constraints on the foreign key, foreign key can be null and duplicate. while creating a table / altering the table, if you add any constrain of uniqueness or not null then only it will not allow the null/ duplicate values.
What is a null foreign key?
A null is the absence of a value. In your case, a value would be a foreign key that references a primary key elsewhere. A null would indicate the absence of a value, just as it always does. At the next level of abstraction, foreign keys represent relationships. Nullable foreign keys represent optional relationships.
Can a foreign key be the only primary key?
The Primary Key column in a table cannot have Null values and should always have unique values. But the Foreign Key in the table can contain Null values and also can have duplicate values . A table can have only one Primary Key whereas there can be more than one Foreign Key for a 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.