How to create foreign key in sql
How do I create a foreign key in MySQL?
Following are the syntax of the ALTER TABLE statement to add a foreign key in the existing table:
- ALTER TABLE table_name.
- ADD [CONSTRAINT [symbol]] FOREIGN KEY.
- [index_name] (column_name, )
- REFERENCES table_name (column_name,)
- ON DELETE referenceOption.
- ON UPDATE referenceOption.
How do you insert a foreign key?
If you are inserting data into a dependent table with foreign keys:
- Each non-null value you insert into a foreign key column must be equal to some value in the corresponding parent key of the parent table.
- If any column in the foreign key is null, the entire foreign key is considered null.
Can foreign key be null?
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 a primary key be a foreign key?
Foreign keys are almost always “Allow Duplicates,” which would make them unsuitable as Primary Keys. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.