Quick Answer: Can Foreign Key Be Null?

Can foreign key be null with example?

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).

Null by definition is not a value..

Can foreign key have duplicate values?

Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them to contain NULL values. Indexes aren’t automatically created for foreign keys; however, as a DBA, you can define them. A table is allowed to contain more than one foreign key.

Can a alternate key be null?

Yes, alternate keys can be null and there can be multiple with null values. You’d only see an error if the value that was entered was duplicated.

Can foreign key be not unique?

InnoDB allows a foreign key constraint to reference a non-unique key. This is an InnoDB extension to standard SQL. However, there is a pratical reason to avoid foreign keys on non-unique columns of referenced table. … You are advised to use foreign keys that reference only UNIQUE (including PRIMARY) and NOT NULL keys.

Can a unique constraint be null?

Details. You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value. This means that it’s possible to insert rows that appear to be duplicates if one of the values is NULL .

Is unique key can be null?

You can only have one primary key per table, but multiple unique keys. Similarly, a primary key column doesn’t accept null values, while unique key columns can contain one null value each.

What is foreign key violation?

It causes violation only if the tuple in relation 1 is deleted which is referenced by foreign key from other tuples of table 2 in the database, if such deletion takes place then the values in the tuple of the foreign key in table 2 will become empty, which will eventually violate Referential Integrity constraint.

Why is my foreign key null?

A: Whether or not a column can contain a NULL value is determined by the presence or absence of a NOT NULL constraint. This is entirely independent of whether the column is referenced in a foreign key constraint. The value in the column is NULL because that’s the value that was assigned when the row was inserted.

Can a foreign key be null mysql?

A foreign key constraint on a stored generated column cannot use CASCADE , SET NULL , or SET DEFAULT as ON UPDATE referential actions, nor can it use SET NULL or SET DEFAULT as ON DELETE referential actions.

Is foreign key NOT NULL by default?

A foreign key attribute can contain NULL values as well. … When a FOREIGN KEY constraint is added to an existing column or columns in the table SQL Server, by default checks the existing data in the columns to ensure that all values, except NULL, exist in the column(s) of the referenced PRIMARY KEY or UNIQUE constraint.

Why is not null used?

By default, a column can hold NULL values. The NOT NULL constraint enforces a column to NOT accept NULL values. This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.

Can foreign key be null Postgres?

We can insert null value into the foreign Key column , foreign key constraint did not block for inserting the null value if column allow null value insertion.. Even you can insert multiple null value into the same column..

Can foreign key be null in DBMS?

A foreign key makes its table dependent on another table called a parent table. … A foreign key containing null values cannot match the values of a parent key, since a parent key by definition can have no null values. However, a null foreign key value is always valid, regardless of the value of any of its non-null parts.

Can a primary key be a foreign key?

Yes, it is legal to have a primary key being a foreign key. This is a rare construct, but it applies for: a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2017, such a database would be odd).

Can primary key have NULL values?

The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values.