Can a table only have a foreign key?
Yes, it is legal to have a primary key being a foreign key. This is a rare construct, but it applies for: a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2017, such a database would be odd).
What is restrict in foreign key?
RESTRICT : Rejects the delete or update operation for the parent table. Specifying RESTRICT (or NO ACTION ) is the same as omitting the ON DELETE or ON UPDATE clause. The MySQL Server rejects the delete or update operation for the parent table if there is a related foreign key value in the referenced table.
Does a foreign key have to point to a primary key?
A foreign key must refer to an entire primary key, and not just part of it. Consider a Department table with a primary key of company_name + department_name. An Employee table should only refer to both attributes and not to department_name alone.
When to enable a FOREIGN KEY constraint in SQL Server?
If any data violates the constraint, the constraint won’t be enabled and you’ll get an error. This is good, because it enforces referential integrity. When you create a new foreign key constraint, this is the default setting. However, when you enable an existing constraint (as we’re doing here), it’s not the default setting.
Can a table in a foreign key relationship be changed?
A table in a foreign key relationship cannot be altered to use another storage engine. To change the storage engine, you must drop any foreign key constraints first. For information about how the MySQL implementation of foreign key constraints differs from the SQL standard, see Section 1.7.2.3, “FOREIGN KEY Constraint Differences” .
Can a column have a foreign key reference to itself?
(A column cannot have a foreign key reference to itself.) In these cases, a “child table record” refers to a dependent record within the same table. MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan.
Can a child table record have a foreign key reference?
MySQL supports foreign key references between one column and another within a table. (A column cannot have a foreign key reference to itself.) In these cases, a “child table record” refers to a dependent record within the same table.