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Why foreign keys are allowed to have NULL values?
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 foreign key have empty values?
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).
Is a foreign key mandatory?
Since the purpose of the foreign key is to identify a particular row of referenced table, it is generally required that the foreign key is equal to the candidate key in some row of the primary table, or else have no value (the NULL value.).
Can a foreign key have a default value equal to 0?
Yes, you can define a column with a default value of 0 as a Foreign Key. However, for the constraint to work, you would need to have a row in the source table with a value of 0 as well. Example:
What is a foreign key in MySQL database?
A foreign key is a rule that makes sure that you use a valid value for the column(s) involved. If you want a default value for a in CHILD you can add another constraint (I’m not sure why default values are considered to be constraints):
Can a column have a default value of 0?
Yes, you can define a column with a default value of 0 as a Foreign Key. However, for the constraint to work, you would need to have a row in the source table with a value of 0 as well.
When to make foreign key AUTO INCREMENT in MySQL?
When I make the foreign key auto increment in MySQL then I can enter data, but it is not the way it should work. May be it can be acceptable for one to one relationship. Shouldn’t the connected table get the value from Master table? or is there sth else I’m missing here? I’d really appreciate any suggestions. Have a nice day.