A pizza box is the rectangular-shaped box in which a computer server is enclosed. It is a horizontally placed framework, usually with a number of similar servers along with it. The term ‘pizza box’ is used because the casing of the server closely resembles the thin size and shape of a pizza box, hence the name.
Referential integrity is a relational database concept, which states that table relationships must always be consistent. In other words, any foreign key field must agree with the primary key that is referenced by the foreign key. Thus, any primary key field changes must be applied to all foreign keys, or not at all. The same restriction also applies to foreign keys in that any updates (but not necessarily deletions) must be propagated to the primary parent key.
Consider a bank database, which contains two tables:
To uniquely identify each customer/accountholder in the CUSTOMER_MASTER table, a primary key column named CUSTOMER_ID is created.
To identify a customer and bank account relationship in the ACCOUNTS_MASTER table, an existing customer in the CUSTOMER_MASTER table must be referenced. Thus, the CUSTOMER_ID column - also created in the ACCOUNTS_MASTER table - is a foreign key. This column is special because its values are not newly-created. Rather, these values must reference existing and identical values in the primary key column of another table, which is the CUSTOMER_ID column of the CUSTOMER_MASTER table.
Referential integrity is a standard that means any CUSTOMER_ID value in the CUSTOMER_MASTER table may not be edited without editing the corresponding value in the ACCOUNTS_MASTER table. For example, if Andrew Smith’s customer ID is changed in the CUSTOMER_MASTER table, this change also must be applied to the ACCOUNTS_MASTER table, thus allowing Andrew Smith’s account information to link to his customer ID.
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