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A foreign key is a set of attributes in a table that refers to the primary key of another table, linking these two tables. In the context of relational databases, a foreign key is subject to an inclusion dependency constraint that the tuples consisting of the foreign key attributes in one relation, R, must also exist in some other (not necessarily distinct) relation, S; furthermore that those ...
A change which ensures that a referenced row exists within another table and/or that ensures that a row which is no longer needed is removed appropriately. Methods of Referential Integrity Refactoring category: Add Foreign Key Constraint; Add Trigger for Calculated Column; Drop Foreign Key Constraint; Introduce Cascading Delete; Introduce Hard ...
A table (called the referencing table) can refer to a column (or a group of columns) in another table (the referenced table) by using a foreign key. The referenced column(s) in the referenced table must be under a unique constraint, such as a primary key. Also, self-references are possible (not fully implemented in MS SQL Server though [5]).
A foreign key is one side of the relationship that shows a row or multiple rows, with one of those rows being the primary key already listed on the first table. This is also called a foreign key constraint, which is important to keep data from being duplicated and have relationships within the database stay reliable as more information is added.
The DROP statement destroys an existing database, table, index, or view. A DROP statement in SQL removes a component from a relational database management system (RDBMS). The types of objects that can be dropped depends on which RDBMS is being used, but most support the dropping of tables , users , and databases .
TRUNCATE TABLE cannot be used when a foreign key references the table to be truncated, since TRUNCATE TABLE statements do not fire triggers. This could result in inconsistent data because ON DELETE/ON UPDATE triggers would not fire. In some computer systems, TRUNCATE TABLE resets the count of an Identity column back to the identity's seed.
Foreign key – referential constraint between two tables. Primary key – Superkey – set of attributes of a relation variable for which it holds that in all relations assigned to that variable, there are no two distinct tuples (rows) that have the same values for the attributes in this set. Surrogate key – unique identifier in a database ...
Indexes are used to police database constraints, such as UNIQUE, EXCLUSION, PRIMARY KEY and FOREIGN KEY. An index may be declared as UNIQUE, which creates an implicit constraint on the underlying table.