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Before defines the second type of integrity constraint, viz., Entity Integrity, we should be well-known with the concept of NULL.
Mostly, NULL is intended as a basis for dealing with the trouble of missing information. This type of situation is frequently encountered in the actual world. For instance, historical records sometimes have entries such as "Date of birth unknown", or police records may contain the entry "Present whereabouts unknown." Therefore it is necessary to have some way of dealing with such concepts in database systems.
Therefore Codd proposed an approach to this issue that creates use of special markers known as NULL to show such missing information.
A given attribute in the relation may or may not be allowed to include NULL. However, can the Primary key or any of its components (in case of the primary key is a composite key) contain a NULL? To answer this Query an Entity Integrity Rule states: No component of the primary key of a relation is permitted to accept NULL. In different words, the definition of each attribute involved in the primary key of any basic relation have to explicitly or implicitly contain the specifications of NULL NOT ALLOWED.
What is a Phantom Deadlock? In distributed deadlock detection, the delay in propagating local information might cause the deadlock detection algorithms to recognize deadlocks
Sequences Sequences: Automatically produce unique numbers Are sharable Are typically used to make a primary key value Replace application code Speed up t
A Relation Schema represented by R(A1, A2, ..., An) is made up of the relation name the list of attributes and R Ai that it contains. A relation is described as a set of tuples
Construct an ER diagram for a hospital along with a set of patients and a set of medical doctors Associate with every patient a log of the several tests and examinations conducted.
As with other software, databases can potentially be used for unethical purposes. As a database developer, and a consumer, you should recognize database misuse, and how it may affe
Objectives After reading this part, you should be able to: Modify, create, and delete database schema objects; Update database using SQL command; Retrieve data f
What is Outer join? They can be employed while we want to keep all the tuples in R, or all those in S or all those in both relations in the result of the JOIN in spite of of w
What are called multilevel indices? Indices with two or more levels are known as multilevel indices.
Backward Recovery (UNDO): In this system the uncommitted changes made by a transaction to a database are undone. Instead the system is reset to the last consistent state of databa
Define the five basic operators of relational algebra? Five basic operators of relational algebra are: 1. Union (∪∪∪∪) - Selects tuples which are in either P or Q or in both
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