Chapter 5 Normalization of Database Tables Database Systems: Design, Implementation, and Management Peter Rob & Carlos Coronel

In this chapter, you will learn: What normalization is and what role it plays in database design About the normal forms 1NF, 2NF, 3NF, BCNF, and 4NF How normal forms can be transformed from lower normal forms to higher normal forms That normalization and E-R modeling are used concurrently to produce a good database design That some situations require denormalization to generate information efficiently

Database Tables and Normalization Normalization is a process for assigning attributes to entities. It reduces data redundancies helps eliminate data anomalies. produces controlled redundancies to link tables Normalization works through a series of stages called normal forms: First normal form (1NF) Second normal form (2NF) Third normal form (3NF) Fourth normal form (4NF) The highest level of normalization is not always desirable.

Database Tables and Normalization The Need for Normalization Case of a Construction Company Building project -- Project number, Name, Employees assigned to the project. Employee -- Employee number, Name, Job classification The company charges its clients by billing the hours spent on each project. The hourly billing rate is dependent on the employee’s position. Periodically, a report is generated. Table 5.1 The easiest way to generate the required report might seem to be a table whose contents correspond to the reporting requirements. Figure 5.1

Database Tables and Normalization Need for Normalization: Problems with the Figure 5.1 The project number is intended to be a primary key, but it contains nulls. The table displays data redundancies. The table entries invite data inconsistencies. The data redundancies yield the following anomalies: Update anomalies. (modify JOB_CLASS for Employee 105) Insertion anomalies. (a new Employee not yet assigned) Deletion anomalies. ( Employee 103 quits)

Database Tables and Normalization Conversion to 1NF Repeating groups – a group of multiple entries can exist for any single key attribute occurrence. Repeating groups must be eliminated Any project number (PROJ_NUM) can have a group of several data entries. A relational table must not contain repeating groups.

Step 2: Identify the Primary Key Step 1: Eliminate the Repeating Groups – Repeating groups can be eliminated by adding the appropriate entry in at least the primary key column(s). Step 2: Identify the Primary Key Uniquely identifies attribute values (rows) Combination of PROJ_NUM and EMP_NUM

Database Tables and Normalization Step 3: Identify all Dependencies Dependency Diagram The primary key components are bold, underlined, and shaded in a different color. The arrows above entities indicate all desirable dependencies ( dependencies based on PK ) The arrows below the dependency diagram indicate less desirable dependencies – partial dependencies ( dependencies based on only a part of PK ) transitive dependencies ( nonprime attribute → nonprime attribute ) Prime attribute = Key attribute Nonprime attribute = Nonkey attribute

Database Tables and Normalization EMP_NUM → EMP_NAME, JOB_CLASS_, CHG_HOUR PROJ_NUM → PROJ_NAME JOB_CLASS → CHG_HOUR

Database Tables and Normalization 1NF Definition The term first normal form (1NF) describes the tabular format in which: All the key attributes are defined. There are no repeating groups in the table. Each row/col intersection can contain one and only one value, not set of values. All attributes are dependent on the primary key. All relational tables satisfy the 1NF requirements. 1NF Drawback Partial dependencies (EMP_NUM → EMP_NAME, JOB_CLASS_, CHG_HOUR) →→ data redundancies →→ data anomalies

Database Tables and Normalization Conversion to 2NF Step 1: Identify All Key Components Writing each key component on a separate line, and then writing the original key on the last line and PROJ_NUM EMP_NUM PROJ_NUM, EMP_NUM Step 2: Identify the Dependent Attributes Writing the dependent attributes after each new key. PROJECT ( PROJ_NUM, PROJ_NAME) EMPLOYEE ( EMP_NUM, EMP_NAME, JOB_CLASS, CHG_HOUR) ASSIGN ( PROJ_NUM, EMP_NUM, HOURS)

Database Tables and Normalization 2NF Definition A table is in 2NF if: It is in 1NF and It includes no partial dependencies; that is, no attribute is dependent on only a portion of the primary key. A table whose primary key is not composite must automatically be in 2NF. BECAUSE a partial dependency can exist only if a table has a composite primary key It is still possible for a table in 2NF to exhibit transitive dependency; that is, one or more attributes may be functionally dependent on nonkey attributes. 2NF Drawback Transitive dependencies (JOB_CLASS → CHG_HOUR) →→ data redundancies →→ data anomalies

Database Tables and Normalization Conversion to 3NF Create a separate table with attributes in a transitive functional dependence relationship. Step 1: Identify Each New Determinant JOB_CLASS Step 2: Identify the Dependent Attributes JOB_CLASS → CHG_HOUR Step 3: Remove the Dependent Attributes from Transitive Dependencies EMPLOYEE (EMP_NUM, EMP_NAME, JOB_CLASS) JOB (JOB_CLASS, CHG_HOUR) PROJECT (PROJ_NUM, PROJ_NAME) ASSIGN (PROJ_NUM, EMP_NUM, HOURS)

Database Tables and Normalization 3NF Definition A table is in 3NF if: It is in 2NF and It contains no transitive dependencies.

Improving the Design Table structures are cleaned up to eliminate the troublesome initial partial and transitive dependencies Normalization cannot, by itself, be relied on to make good designs It is valuable because its use helps eliminate data redundancies

Improving the Design (continued) The following changes were made: PK assignment Naming conventions Attribute atomicity Adding attributes Adding relationships Refining PKs Maintaining historical accuracy Using derived attributes

Improving the Design Adding relationships 3NF Project’s manager EMP_NUM as a FK in PROJECT. Project manager 3NF

Improving the Design PK assignment Naming conventions 2NF JOB_CODE JOB_CHG_HOUR JOB_CLASS >> JOB_DESCRIPTION 2NF

Improving the Design 3NF Attribute atomicity EMP_NAME >> EMP_LNAME,EMP_FNAME,EMP_INITIAL Adding attributes EMP_HIREDATE JOB_CLASS

3NF Improving the Design Refining PKs (EMP_NUM+PROJ_NUM) >> ASSIGN_NUM Maintaining historical accuracy ASSIGN_CHG_HOUR <<>> JOB_CHG_HOUR Using derived attributes ASSIGN_CHARGE = ASSIGN_HOURS × ASSIGN_CHG_HOUR 3NF

The Completed Database

The Completed Database (continued)

Database Tables and Normalization Boyce-Codd Normal Form (BCNF) A table is in Boyce-Codd normal form (BCNF) if every determinant in the table is a candidate key. (A determinant is any attribute whose value determines other values with a row.) If a table contains only one candidate key, the 3NF and the BCNF are equivalent. BCNF can be violated only if the table contains more than one candidate key BCNF is a special case of 3NF. Figure 5.7 illustrates a table that is in 3NF but not in BCNF. Figure 5.8 shows how the table can be decomposed to conform to the BCNF form.

A Table That Is In 3NF But Not In BCNF A + B → C, D C → B : Not transitive dependencies (A nonkey attribute is the determinant of a key attribute) → → 3NF C : Not candidate key → → Not In BCNF Figure 5.7

A + B → C, D C → B A + C → B, D C → B Figure 5.8 A + C → D C → B The Decomposition of a Table Structure to meet BCNF Requirements A + B → C, D C → B Change the PK to A+C A + C → B, D C → B 1NF Figure 5.8 A + C → D C → B

Sample Data for a BCNF Conversion

Decomposition into BCNF Figure 5.9

Normalization and Database Design Normalization should be part of the design process E-R Diagram provides macro view Normalization provides micro view of entities Focuses on characteristics of specific entities A micro view of the entities within the ER diagram Difficult to separate normalization from E-R diagramming Two techniques should be used concurrently

Normalization and Database Design Database Design and Normalization Example: (Construction Company) Summary of Operations: The company manages many projects. Each project requires the services of many employees. An employee may be assigned to several different projects. Some employees are not assigned to a project and perform duties not specifically related to a project. Some employees are part of a labor pool, to be shared by all project teams. Each employee has a (single) primary job classification. This job classification determines the hourly billing rate. Many employees can have the same job classification.

Normalization and Database Design Two Initial Entities: PROJECT (PROJ_NUM, PROJ_NAME) [ 3NF ] EMPLOYEE ( EMP_NUM, EMP_LNAME, EMP_FNAME, EMP_INITIAL, JOB_DESCRIPTION, JOB_CHG_HOUR) No partial dep. Transitive dep. JOB_DESCRIPTION → JOB_CHG_HOUR [ 2NF ]

Normalization and Database Design Three Entities After Transitive Dependency Removed PROJECT (PROJ_NUM, PROJ_NAME) EMPLOYEE ( EMP_NUM, EMP_LNAME, EMP_FNAME, EMP_INITIAL, JOB_CODE) JOB ( JOB_CODE, JOB_DESCRIPTION, JOB_CHG_HOUR) EMPLOYEE

The Modified ERD For A Contracting Company Because the normalization process yields an additional entity (JOB), we modify the initial ERD. M Is held by

Normalization and Database Design Creation of the Composite Entity ASSIGNMENT ASSIGNMENT Is held by The Final ( Implementable) ERD for the Contracting Company

Normalization and Database Design Attribute ASSIGN_HOUR is assigned to the composite entity ASSIGN. “Manages” relationship is created between EMPLOYEE and PROJECT. PROJECT (PROJ_NUM, PROJ_NAME, EMP_NUM) ASSIGNMENT (ASSIGN_NUM, ASSIGN_DATE , ASSIGN_HOURS, ASSIGN_CHG_HOUR , ASSIGN_CHARGE, EMP_NUM, PROJ_NUM) EMPLOYEE (EMP_NUM, EMP_LNAME, EMP_FNAME, EMP_INITIAL, EMP_HIREDATE, JOB_CODE) JOB (JOB_CODE, JOB_DESCRIPTION, JOB_CHG_HOUR) Manages

Higher-Level Normal Forms In some databases, multiple multivalued attributes exist 4NF Definition A table is in 4NF if it is in 3NF and has no multiple sets of multivalued dependencies.

Higher-Level Normal Forms An employee can have multiple assignments and can also be involved in multiple service organization. EMP_SERVICE (volunteer work) and EMP_ASSIGN (assigned project) each may have many different values. The table contain two sets of multivalued dependencies. Independent 1

A Set of Tables in 4NF The solution is to eliminate the problems caused by independent multivalued dependencies.

Denormalization Creation of normalized relations is important database design goal Processing requirements should also be a goal If tables decomposed to conform to normalization requirements Number of database tables expands Joining larger number of tables takes additional disk input/output (I/O) operations and processing logic Reduces system speed

Denormalization Normalization is only one of many database design goals. Normalized (decomposed) tables require additional processing, ( Join : additional I/O operations ) reduce system speed. CUSTOMER( CUS_NUM, CUS_NAME, … , ZIP_CODE, CITY) Is it really practical? ZIP(ZIP_CODE, CITY) Some degree of denormalization →→ increase processing speed transitive dep.

Denormalization Normalization purity is often difficult to sustain in the modern database environment. The conflict between design efficiency, information requirements, and processing speed are often resolved through compromises that include denormalization. In fact, we used a 2NF structure in the JOB table (Fig.5.6) to decrease the likelihood of referential integrity violations. JOB table Original PK: JOB_CLASS New PK: JOB_CODE EMPLOYEE table Original FK: JOB_CLASS New FK: JOB_CODE Use denormalization cautiously.

The Initial 1NF Structure

Identifying the Possible PK Attributes

Table Structures Based On The Selected PKs Foreign key