1. LIS 384K.11 Database-Management Principles and Applications
Tables and Their Components
R. E. Wyllys
Copyright © 2002 by R. E. Wyllys
Last revised 2002 Jan 28
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

2. Types of Keys1 Superkey Candidate Key Primary Key Secondary Key
Foreign Key
1For a detailed discussion of keys, see the Course Note entitled Key Concepts.
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

3. Types of Keys Superkey Candidate Key
A key (i.e., a column or combination of columns containing no null values and no duplicate values) that identifies each row uniquely.
Candidate Key
For Rob and Coronel, a minimal superkey (i.e., a superkey with no subsets that are themselves superkeys)
For most writers, any superkey (i.e., any key that identifies each row uniquely)
Note: Most writers equate "candidate keys" and "superkeys", and use only the term "candidate keys"
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

4. Types of Keys Primary Key
The candidate key (superkey) that has been selected as the preferred key for identifying records in a particular table
Provides a unique way to identify each record
Can be obvious from the structure of the table. If there is no easy natural choice, you can add a column containing a unique identifier.
May consist of one or more columns; multiple columns need not be adjacent
A single-column key is called a simple key
A multiple-column key is called a composite key
May consist of the entire record (especially with two-column tables, which occur often in the development of RDBs)
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

5. Types of Keys Secondary Key Foreign Key
A column that is used to aid in the retrieval of information from a table. A secondary key is not required to have unique values in each of its rows,
Foreign Key
A column that is used to aid in the retrieval of information from one table (i.e., is a secondary key) and that is also the primary key in another table. Foreign keys are a major tool in RDBs.
Because of referential integrity, a foreign key should have only values that occur in the table in which it is the primary key.
All foreign keys are secondary keys, but not all secondary keys are foreign keys.
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

6. Types of Fields Character Numeric Logical Image Object
LBO (Large Binary Object)
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

7. Types of Fields
Character: Alphanumeric and punctuation. Character fields can include numerals, but calculations cannot be performed on the contents of the field even though they look like numbers.
Examples: Names, product codes, SSNs, ZIPCodes
Sometimes further differentiated between memo fields (short) and text fields (long)
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

8. Types of Fields
Numeric: Arithmetic operations can be performed on the contents of the field
Numbers (sometimes further differentiated among integers [short, long], fixed-decimal point numbers, floating-point numbers)
Dates (addition and subtraction only)
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

9. Types of Fields
General Rule for Numeric Fields: Never assign a field a numeric data type unless you intend to perform mathematical calculations on its contents.
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

10. Types of Fields
Logical: True-False (T, F), Boolean (0, 1), Yes-No (Y, N)
Image: Used for relatively small graphics files (e.g., under 100KB)
Object: An object is a package of data together with program code that performs certain functions on the data
Large Binary Object: Used for very large items, such as multi-megabyte graphic, audio, and video files; the RDB typically merely stores pointers to the locations of these large files
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications

11. The World is Full of Files
GSLIS - The University of Texas at Austin
LIS 384K.11, Database-Management Principles and Applications