No they can not
why there is no answer yet
Yes Foreign key can have null value.
No. A Primary Key is the Unique identifier and cannot be null and cannot be duplicate.
No.
No. A primary key requires a value because it is the field that all other values in the row depend on.
referential integrity constraint
The entity integrity constraint states that no primary key value can be null. This is because the primary key value is used to identify individual tuples in a relation. Having null value for the primary key implies that we cannot identify some tuples. This also specifies that there may not be any duplicate entries in primary key column key row.
1) Entity Integrity: In a base relation, no attribute of a primary key can be null. 2) Referential Integrity: If foreign key exists in a relation, either foreign key value must match a candidate key value of some tuple in its home relation or foreign key value must be wholly null
yes .a foreign key can have null values
By shifting the values in an array, you are moving a key's value to the previous key. The very first key's value is obliterated. By shifting all values in the array, all keys will have a value of NULL. Unsetting a variable is entirely different -- performing a variable unsetting causes the variable to have a value of NULL, as if it was never set.
No unique can not be nulll....unique must have any unique value for unique identification.,...
In databases: Null Value: Represents the absence of a value or an unknown value. It indicates that the data is missing or not applicable. Not Null Value: Indicates that a field contains a valid, defined value. It means the data is present and has been explicitly set.