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Can a foreign key contain NULL values?
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. A table can have many foreign keys.
Is index required on foreign key?
It is highly recommended to create an index on the foreign key columns, to enhance the performance of the joins between the primary and foreign keys, and also reduce the cost of maintaining the relationship between the child and parent tables.
How to allow null values on foreign keys using EF?
This foreign key is per definition nullable, unless you add the [Required] attribute to the navigation property. For example, if you want to make the foreign key in the User entity referring to (“pointing to”) Computer mandatory, the declaration [Key, ForeignKey (“idComputer”), Required] would do that.
Why are there no null entries in the index?
I googled and found out there are no null entries in indexes, thus the first query can’t use the index. My question is simple: why there aren’t null entries in indexes? By default, relational databases ignore NULL values (because the relational model says that NULL means “not present”).
Why are null values ignored in a relational database?
By default, relational databases ignore NULL values (because the relational model says that NULL means “not present”). So, Index does not store NULL value, consequently if you have null condition in SQL statement, related index is ignored (by default).
Can a null value be set to a column?
NULL refers to the absence of value, so when you set a column to NULL, it wouldn’t make sense to try to enforce constraints on that. The above works but this does not. Note the ON DELETE CASCADE Yes, the value can be NULL, but you must be explicit.