If dependent rows in referencing tables still exist, those references have to be considered. Likewise, foreign keys can be defined as part of the CREATE TABLE SQL statement.īecause the database management system enforces referential constraints, it must ensure data integrity if rows in a referenced table are to be deleted (or updated). Omitting the column list in the REFERENCES clause implies that the foreign key shall reference the primary key of the referenced table. The syntax to add such a constraint to an existing table is defined in SQL:2003 as shown below. Therefore, cascading relationships between tables can be established using foreign keys.Ī foreign key is defined as an attribute or set of attributes in a relation whose values match a primary key in another relation. Each foreign key is enforced independently by the database system. In database management systems, this is often accomplished by linking a first and second reference to the same table.Ī table may have multiple foreign keys, and each foreign key can have a different parent table. Such a foreign key is known in SQL:2003 as a self-referencing or recursive foreign key. the foreign key refers back to the same table. In addition, the child and parent table may, in fact, be the same table, i.e. In this case, the relationship between the two tables is called a one to many relationship between the referencing table and the referenced table. Multiple rows in the referencing (or child) table may refer to the same row in the referenced (or parent) table. Īnother important part of database design is database normalization, in which tables are broken apart and foreign keys make it possible for them to be reconstructed.
One important part of database design is making sure that relationships between real-world entities are reflected in the database by references, using foreign keys to refer from one table to another.
#PHPMYADMIN FOREIGN KEY UPDATE#
Many real world databases work around this problem by 'inactivating' rather than physically deleting master table foreign keys, or by complex update programs that modify all references to a foreign key when a change is needed.įoreign keys play an essential role in database design.
However, this can no longer be assumed if the ORDER table is not kept up to date when rows of the CUSTOMER table are deleted or the ID column altered, and working with these tables may become more difficult. Because the primary key of a table must be unique, and because CUSTOMERID only contains values from that primary key field, we may assume that, when it has a value, CUSTOMERID will identify the particular customer which placed the order. To reflect this in the database, a foreign key column is added to the ORDER table (e.g., CUSTOMERID), which references the primary key of CUSTOMER (e.g. Suppose the business requires that each order must refer to a single customer. įor example, consider a database with two tables: a CUSTOMER table that includes all customer data and an ORDER table that includes all customer orders. īecause violations of these constraints can be the source of many database problems, most database management systems provide mechanisms to ensure that every non-null foreign key corresponds to a row of the referenced table. This rule is called a referential integrity constraint between the two tables.
Since the purpose of the foreign key is to identify a particular row of referenced table, it is generally required that the foreign key is equal to the candidate key in some row of the primary table, or else have no value (the NULL value. The value or combination of values of candidate key attributes for any tuple cannot be duplicated for any other tuple in that relation. In database relational modeling and implementation, a candidate key is a set of zero or more attributes, the values of which are guaranteed to be unique for each tuple (row) in a relation. The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.
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