Välisvõtmetega

Välisvõtmetega refers to the concept of foreign keys in relational database management. A foreign key is a column or a set of columns in one table that references the primary key of another table. Its primary purpose is to establish and enforce a link between two tables, ensuring referential integrity.

When a foreign key constraint is defined, the database system verifies that any value entered into the foreign key column(s) must already exist in the primary key column(s) of the referenced table. This prevents orphaned records, where a record in one table refers to a non-existent record in another. For example, in a database with "Customers" and "Orders" tables, the "CustomerID" column in the "Orders" table would typically be a foreign key referencing the "CustomerID" primary key in the "Customers" table. This ensures that every order is associated with a valid customer.

Foreign keys play a crucial role in data consistency and the logical structure of a relational database. They facilitate operations like joins, allowing data from related tables to be combined and queried efficiently. Common actions associated with foreign keys include ON DELETE and ON UPDATE clauses, which define how the database should behave when a referenced primary key is deleted or updated. These actions can include CASCADE (propagating the change), SET NULL (setting the foreign key to null), or RESTRICT (preventing the change).