Can a foreign key be null in a 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. A foreign key value is null if any part is null.
Can foreign key constraint have null and duplicate 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).
How to add a not null foreign key column?
2 Answers 2. Create the column, but allow . Populate the column with the correct data from the foreign key table. Alter the column add not null. Add the foreign key constraint. You could add the column with a default, then copy over the correct data, then drop the default constraint.
Is there way to drop column with foreign key?
I am not allowed to alter the DB in any way other than removing the column. Your DROP FOREIGN KEY syntax is using the wrong key name. It’s trying to drop your “plain” index on the home_lang field. It’s NOT the foreign key itself. Use this given below query to find the name of the foreign key.
Is the foreign key in the referencing table unique?
In the original table (“referenced table”), the foreign key should be unique and non- . In fact, it should almost always be the primary key of that table. In the referencing table, the referencing column should only be declared not- NULL if a value is always required.
When is a foreign field null in SQL?
That depends on whether your business model requires each customer to declare their home country or not. If the model allows you to do business with someone who does not have a home country, or does not wish to reveal the home country to you, then the field should be NULLable.