Can two records have the same foreign key?
You can actually have the same nameless FK constraint definition text multiple times in a table and in multiple tables, each for a different FK constraint.
Is a foreign key necessary for join?
A primary key is not required. A foreign key is not required either. You can construct a query joining two tables on any column you wish as long as the datatypes either match or are converted to match.
Can we create a foreign key without a primary key?
If you really want to create a foreign key to a non-primary key, it MUST be a column that has a unique constraint on it.
Can a SQL table exist without primary key?
Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key. (The data foreign key column need not exist in the primary key column in the first table.)
How to join two tables in MySQL using foreign key?
I need to show the table employees, but instead of department_id (showing the IDs of the departments) I need to show the actual departments name, so in place of department_id, i need to place departments.department. How should I do this? You just have to use a inner join between your two tables like this:
How is a foreign key relationship defined in MySQL?
A foreign key relationship involves a parent table that holds the initial column values, and a child table with column values that reference the parent column values. A foreign key constraint is defined on the child table.
How are foreign key constraints checked in MySQL?
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. Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT .
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.