Problems with NULL Values and Dangling Tuples
16.4.1 Problems with NULL Values and Dangling Tuples
We must carefully consider the problems associated with NULL s when designing a relational database schema. There is no fully satisfactory relational design theory as yet that includes NULL values. One problem occurs when some tuples have NULL values for attributes that will be used to join individual relations in the decomposi- tion. To illustrate this, consider the database shown in Figure 16.2(a), where two relations EMPLOYEE and DEPARTMENT are shown. The last two employee tuples—
‘Berger’ and ‘Benitez’—represent newly hired employees who have not yet been assigned to a department (assume that this does not violate any integrity con- straints). Now suppose that we want to retrieve a list of ( Ename , Dname ) values for
all the employees. If we apply the NATURAL JOIN operation on EMPLOYEE and DEPARTMENT (Figure 16.2(b)), the two aforementioned tuples will not appear in the result. The OUTER JOIN operation, discussed in Chapter 6, can deal with this problem. Recall that if we take the LEFT OUTER JOIN of EMPLOYEE with DEPARTMENT , tuples in EMPLOYEE that have NULL for the join attribute will still appear in the result, joined with an imaginary tuple in DEPARTMENT that has NULL s for all its attribute values. Figure 16.2(c) shows the result.
In general, whenever a relational database schema is designed in which two or more relations are interrelated via foreign keys, particular care must be devoted to watch- ing for potential NULL values in foreign keys. This can cause unexpected loss of information in queries that involve joins on that foreign key. Moreover, if NULL s occur in other attributes, such as Salary , their effect on built-in functions such as SUM and AVERAGE must be carefully evaluated.
A related problem is that of dangling tuples, which may occur if we carry a decom- position too far. Suppose that we decompose the EMPLOYEE relation in Figure 16.2(a) further into EMPLOYEE_1 and EMPLOYEE_2 , shown in Figure 16.3(a) and
16.3(b). 9 If we apply the NATURAL JOIN operation to EMPLOYEE_1 and EMPLOYEE_2 , we get the original EMPLOYEE relation. However, we may use the alternative representation, shown in Figure 16.3(c), where we do not include a tuple
9 This sometimes happens when we apply vertical fragmentation to a relation in the context of a distrib-
564 Chapter 16 Relational Database Design Algorithms and Further Dependencies
(a) Figure 16.2
EMPLOYEE Issues with NULL-value Ename
joins. (a) Some Smith, John B.
123456789 1965-01-09 731 Fondren, Houston, TX 5 EMPLOYEE tuples have Wong, Franklin T.
333445555 1955-12-08 638 Voss, Houston, TX 5 NULL for the join attrib- Zelaya, Alicia J.
999887777 1968-07-19 3321 Castle, Spring, TX 4 ute Dnum. (b) Result of applying NATURAL JOIN
Wallace, Jennifer S. 987654321 1941-06-20 291 Berry, Bellaire, TX 4 to the EMPLOYEE and Narayan, Ramesh K.
666884444 1962-09-15 975 Fire Oak, Humble, TX 5 DEPARTMENT relations. English, Joyce A.
453453453 1972-07-31 5631 Rice, Houston, TX 5 (c) Result of applying Jabbar, Ahmad V.
987987987 1969-03-29 980 Dallas, Houston, TX 4 LEFT OUTER JOIN to Borg, James E.
888665555 1937-11-10 450 Stone, Houston, TX 1 EMPLOYEE and Berger, Anders C.
DEPARTMENT. Benitez, Carlos M.
999775555 1965-04-26 6530 Braes, Bellaire, TX
NULL
888664444 1963-01-09 7654 Beech, Houston, TX
NULL
DEPARTMENT Dname
Dnum
Dmgr_ssn
Dname Dmgr_ssn Smith, John B.
Ename Ssn
123456789 1965-01-09 731 Fondren, Houston, TX 5 Research 333445555 Wong, Franklin T.
333445555 1955-12-08 638 Voss, Houston, TX 5 Research 333445555 Zelaya, Alicia J.
999887777 1968-07-19 3321 Castle, Spring, TX 4 Administration 987654321 Wallace, Jennifer S. 987654321 1941-06-20 291 Berry, Bellaire, TX
4 Administration 987654321 Narayan, Ramesh K. 666884444 1962-09-15 975 Fire Oak, Humble, TX
5 Research 333445555 English, Joyce A.
453453453 1972-07-31 5631 Rice, Houston, TX 5 Research 333445555 Jabbar, Ahmad V.
987987987 1969-03-29 980 Dallas, Houston, TX 4 Administration 987654321 Borg, James E.
888665555 1937-11-10 450 Stone, Houston, TX 1 Headquarters 888665555
(c)
Dname Dmgr_ssn Smith, John B.
Ename Ssn
123456789 1965-01-09 731 Fondren, Houston, TX 5 Research 333445555 Wong, Franklin T.
333445555 1955-12-08 638 Voss, Houston, TX 5 Research 333445555 Zelaya, Alicia J.
999887777 1968-07-19 3321 Castle, Spring, TX 4 Administration 987654321 Wallace, Jennifer S. 987654321 1941-06-20 291 Berry, Bellaire, TX
4 Administration 987654321 Narayan, Ramesh K. 666884444 1962-09-15 975 Fire Oak, Humble, TX
5 Research 333445555 English, Joyce A.
453453453 1972-07-31 5631 Rice, Houston, TX 5 Research 333445555 Jabbar, Ahmad V.
987987987 1969-03-29 980 Dallas, Houston, TX 4 Administration 987654321 Borg, James E.
888665555 1937-11-10 450 Stone, Houston, TX 1 Headquarters 888665555 Berger, Anders C.
999775555 1965-04-26 6530 Braes, Bellaire, TX NULL NULL NULL
16.4 About Nulls, Dangling Tuples, and Alternative Relational Designs 565
(a) EMPLOYEE_1 Ename
Smith, John B. 123456789 1965-01-09 731 Fondren, Houston, TX Wong, Franklin T.
333445555 1955-12-08 638 Voss, Houston, TX
Zelaya, Alicia J.
999887777 1968-07-19 3321 Castle, Spring, TX
Wallace, Jennifer S.
987654321 1941-06-20 291 Berry, Bellaire, TX
Narayan, Ramesh K. 666884444 1962-09-15 975 Fire Oak, Humble, TX English, Joyce A.
453453453 1972-07-31
5631 Rice, Houston, TX
Jabbar, Ahmad V.
987987987 1969-03-29 980 Dallas, Houston, TX
Borg, James E.
888665555 1937-11-10 450 Stone, Houston, TX
Berger, Anders C.
999775555 1965-04-26 6530 Braes, Bellaire, TX
Benitez, Carlos M. 888665555 1963-01-09 7654 Beech, Houston, TX
(b) EMPLOYEE_2
(c) EMPLOYEE_3
Ssn Dnum
4 The dangling tuple problem. 987654321
4 (a) The relation EMPLOYEE_1 ( includes 666884444
5 all attributes of EMPLOYEE from Figure 16.2 ( a ) except Dnum ) . 453453453
5 (b) The relation EMPLOYEE_2 ( includes 987987987
4 Dnum attribute with NULL values ) . 888665555
1 (c) The relation EMPLOYEE_3 ( includes Dnum attribute but does not include 999775555
NULL tuples for which Dnum has NULL
888664444 NULL
values ) .
in EMPLOYEE_3 if the employee has not been assigned a department (instead of including a tuple with NULL for Dnum as in EMPLOYEE_2 ). If we use EMPLOYEE_3 instead of EMPLOYEE_2 and apply a NATURAL JOIN on EMPLOYEE_1 and EMPLOYEE_3 , the tuples for Berger and Benitez will not appear in the result; these are called dangling tuples in EMPLOYEE_1 because they are represented in only one of the two relations that represent employees, and hence are lost if we apply an ( INNER ) JOIN operation.
Parts
» Fundamentals_of_Database_Systems,_6th_Edition
» Characteristics of the Database Approach
» Advantages of Using the DBMS Approach
» A Brief History of Database Applications
» Schemas, Instances, and Database State
» The Three-Schema Architecture
» The Database System Environment
» Centralized and Client/Server Architectures for DBMSs
» Classification of Database Management Systems
» Domains, Attributes, Tuples, and Relations
» Key Constraints and Constraints on NULL Values
» Relational Databases and Relational Database Schemas
» Integrity, Referential Integrity, and Foreign Keys
» Update Operations, Transactions, and Dealing with Constraint Violations
» SQL Data Definition and Data Types
» Specifying Constraints in SQL
» The SELECT-FROM-WHERE Structure of Basic SQL Queries
» Ambiguous Attribute Names, Aliasing, Renaming, and Tuple Variables
» Substring Pattern Matching and Arithmetic Operators
» INSERT, DELETE, and UPDATE Statements in SQL
» Comparisons Involving NULL and Three-Valued Logic
» Nested Queries, Tuples, and Set/Multiset Comparisons
» The EXISTS and UNIQUE Functions in SQL
» Joined Tables in SQL and Outer Joins
» Grouping: The GROUP BY and HAVING Clauses
» Discussion and Summary of SQL Queries
» Specifying General Constraints as Assertions in SQL
» Introduction to Triggers in SQL
» Specification of Views in SQL
» View Implementation, View Update, and Inline Views
» Schema Change Statements in SQL
» Sequences of Operations and the RENAME Operation
» The UNION, INTERSECTION, and MINUS Operations
» The CARTESIAN PRODUCT (CROSS PRODUCT) Operation
» Variations of JOIN: The EQUIJOIN and NATURAL JOIN
» Additional Relational Operations
» Examples of Queries in Relational Algebra
» The Tuple Relational Calculus
» The Domain Relational Calculus
» Using High-Level Conceptual Data Models
» Entity Types, Entity Sets, Keys, and Value Sets
» Relationship Types, Relationship Sets, Roles, and Structural Constraints
» ER Diagrams, Naming Conventions, and Design Issues
» Example of Other Notation: UML Class Diagrams
» Relationship Types of Degree Higher than Two
» Subclasses, Superclasses, and Inheritance
» Constraints on Specialization and Generalization
» Specialization and Generalization Hierarchies
» Modeling of UNION Types Using Categories
» A Sample UNIVERSITY EER Schema, Design Choices, and Formal Definitions
» Data Abstraction, Knowledge Representation, and Ontology Concepts
» ER-to-Relational Mapping Algorithm
» Discussion and Summary of Mapping for ER Model Constructs
» Mapping EER Model Constructs
» The Role of Information Systems
» The Database Design and Implementation Process
» Use of UML Diagrams as an Aid to Database Design Specification 6
» Rational Rose: A UML-Based Design Tool
» Automated Database Design Tools
» Introduction to Object-Oriented Concepts and Features
» Object Identity, and Objects versus Literals
» Complex Type Structures for Objects and Literals
» Encapsulation of Operations and Persistence of Objects
» Type Hierarchies and Inheritance
» Other Object-Oriented Concepts
» Object-Relational Features: Object Database Extensions to SQL
» Overview of the Object Model of ODMG
» Built-in Interfaces and Classes in the Object Model
» Atomic (User-Defined) Objects
» Extents, Keys, and Factory Objects
» The Object Definition Language ODL
» Differences between Conceptual Design of ODB and RDB
» Mapping an EER Schema to an ODB Schema
» Query Results and Path Expressions
» Overview of the C++ Language Binding in the ODMG Standard
» Structured, Semistructured, and Unstructured Data
» XML Hierarchical (Tree) Data Model
» Well-Formed and Valid XML Documents and XML DTD
» XPath: Specifying Path Expressions in XML
» XQuery: Specifying Queries in XML
» Extracting XML Documents from
» Database Programming: Techniques
» Retrieving Single Tuples with Embedded SQL
» Retrieving Multiple Tuples with Embedded SQL Using Cursors
» Specifying Queries at Runtime Using Dynamic SQL
» SQLJ: Embedding SQL Commands in Java
» Retrieving Multiple Tuples in SQLJ Using Iterators
» Database Programming with SQL/CLI Using C
» JDBC: SQL Function Calls for Java Programming
» Database Stored Procedures and SQL/PSM
» PHP Variables, Data Types, and Programming Constructs
» Overview of PHP Database Programming
» Imparting Clear Semantics to Attributes in Relations
» Redundant Information in Tuples and Update Anomalies
» Normal Forms Based on Primary Keys
» General Definitions of Second and Third Normal Forms
» Multivalued Dependency and Fourth Normal Form
» Join Dependencies and Fifth Normal Form
» Inference Rules for Functional Dependencies
» Minimal Sets of Functional Dependencies
» Properties of Relational Decompositions
» Dependency-Preserving Decomposition
» Dependency-Preserving and Nonadditive (Lossless) Join Decomposition into 3NF Schemas
» Problems with NULL Values and Dangling Tuples
» Discussion of Normalization Algorithms and Alternative Relational Designs
» Further Discussion of Multivalued Dependencies and 4NF
» Other Dependencies and Normal Forms
» Memory Hierarchies and Storage Devices
» Hardware Description of Disk Devices
» Magnetic Tape Storage Devices
» Placing File Records on Disk
» Files of Unordered Records (Heap Files)
» Files of Ordered Records (Sorted Files)
» External Hashing for Disk Files
» Hashing Techniques That Allow Dynamic File Expansion
» Other Primary File Organizations
» Parallelizing Disk Access Using RAID Technology
» Types of Single-Level Ordered Indexes
» Some General Issues Concerning Indexing
» Algorithms for External Sorting
» Implementing the SELECT Operation
» Implementing the JOIN Operation
» Algorithms for PROJECT and Set
» Notation for Query Trees and Query Graphs
» Heuristic Optimization of Query Trees
» Catalog Information Used in Cost Functions
» Examples of Cost Functions for SELECT
» Examples of Cost Functions for JOIN
» Example to Illustrate Cost-Based Query Optimization
» Factors That Influence Physical Database Design
» Physical Database Design Decisions
» An Overview of Database Tuning in Relational Systems
» Transactions, Database Items, Read and Write Operations, and DBMS Buffers
» Why Concurrency Control Is Needed
» Transaction and System Concepts
» Desirable Properties of Transactions
» Serial, Nonserial, and Conflict-Serializable Schedules
» Testing for Conflict Serializability of a Schedule
» How Serializability Is Used for Concurrency Control
» View Equivalence and View Serializability
» Types of Locks and System Lock Tables
» Guaranteeing Serializability by Two-Phase Locking
» Dealing with Deadlock and Starvation
» Concurrency Control Based on Timestamp Ordering
» Multiversion Concurrency Control Techniques
» Validation (Optimistic) Concurrency
» Granularity of Data Items and Multiple Granularity Locking
» Using Locks for Concurrency Control in Indexes
» Other Concurrency Control Issues
» Recovery Outline and Categorization of Recovery Algorithms
» Caching (Buffering) of Disk Blocks
» Write-Ahead Logging, Steal/No-Steal, and Force/No-Force
» Transaction Rollback and Cascading Rollback
» NO-UNDO/REDO Recovery Based on Deferred Update
» Recovery Techniques Based on Immediate Update
» The ARIES Recovery Algorithm
» Recovery in Multidatabase Systems
» Introduction to Database Security Issues 1
» Discretionary Access Control Based on Granting and Revoking Privileges
» Mandatory Access Control and Role-Based Access Control for Multilevel Security
» Introduction to Statistical Database Security
» Introduction to Flow Control
» Encryption and Public Key Infrastructures
» Challenges of Database Security
» Distributed Database Concepts 1
» Types of Distributed Database Systems
» Distributed Database Architectures
» Data Replication and Allocation
» Example of Fragmentation, Allocation, and Replication
» Query Processing and Optimization in Distributed Databases
» Overview of Transaction Management in Distributed Databases
» Overview of Concurrency Control and Recovery in Distributed Databases
» Current Trends in Distributed Databases
» Distributed Databases in Oracle 13
» Generalized Model for Active Databases and Oracle Triggers
» Design and Implementation Issues for Active Databases
» Examples of Statement-Level Active Rules
» Time Representation, Calendars, and Time Dimensions
» Incorporating Time in Relational Databases Using Tuple Versioning
» Incorporating Time in Object-Oriented Databases Using Attribute Versioning
» Temporal Querying Constructs and the TSQL2 Language
» Spatial Database Concepts 24
» Multimedia Database Concepts
» Clausal Form and Horn Clauses
» Datalog Programs and Their Safety
» Evaluation of Nonrecursive Datalog Queries
» Introduction to Information Retrieval
» Types of Queries in IR Systems
» Evaluation Measures of Search Relevance
» Web Analysis and Its Relationship to Information Retrieval
» Analyzing the Link Structure of Web Pages
» Approaches to Web Content Analysis
» Trends in Information Retrieval
» Data Mining as a Part of the Knowledge
» Goals of Data Mining and Knowledge Discovery
» Types of Knowledge Discovered during Data Mining
» Market-Basket Model, Support, and Confidence
» Frequent-Pattern (FP) Tree and FP-Growth Algorithm
» Other Types of Association Rules
» Approaches to Other Data Mining Problems
» Commercial Data Mining Tools
» Data Modeling for Data Warehouses
» Difficulties of Implementing Data Warehouses
» Grouping, Aggregation, and Database Modification in QBE
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