2.4 IDENTIFYING KEYS 2.4.1 Primary Key and Entity Integrity An attribute that uniquely identifies one and only one instance of an entity is called a primary key. Sometimes a primary key consists of a combination of attributes. It is referred to as a composite key. The entity integrity rule states […]
Foreign Keys and Referential Integrity – Introduction to Databases
2.4.3 Foreign Keys and Referential Integrity Foreign keys are used to create relationships between Tables. It is an attribute in one Table whose values are required to match those of primary key in another Table. Foreign keys are created to enforce referential integrity, which states that you may not add […]
DEFINE RELATIONSHIPS – Introduction to Databases
2.5 DEFINE RELATIONSHIPS 2.5.1 Connectivity Connectivity refers to the types of relationships that entities can have. Basically, the relationship can be one-to-one, one-to-many and many-to-many. In ER diagrams, these are indicated by placing 1, M orN at one of the two ends of the relationship diagram. Figures 2.2–2.5 illustrate the […]
ER NOTATION – Introduction to Databases
2.6 ER NOTATION There are a number of ER notations available, including Chen’s, Bachman, crow’s foot and a few others. There is no consensus on the symbols and the styles used to draw ERDs. A number of draw-ing tools are available to draw ERDs. These include ER Assistant, Microsoft Visio […]
Third Normal Form (3NF) – Introduction to Databases
2.7.3 Third Normal Form (3NF) A Table is in third normal form (3NF) if it is already in 2NF and every non-key column is non-transitively dependent upon the primary key. In other words, all non-key columns are mutually independent, but at the same time, they are fully dependent upon the […]
DATABASE COMPONENTS IN SOME POPULAR DATABASES – Introduction to Databases
2.8 DATABASE COMPONENTS IN SOME POPULAR DATABASES All databases allow for storage, retrieval and management of the data. Simple databases provide basic services to accomplish these tasks. Many database providers, like Microsoft SQL Server and Oracle, provide additional services that necessitate storing many components in the database other than data. […]
Tables – Introduction to Databases
2.8.1.2 Tables Before you can create a Table in Access, you must create a database container and give it a name with the extension. mdb. Database creation is simple process and is explained in detail with an example later in this chapter. Suffice it to say that a Table is […]
Data Files – Introduction to Databases
2.8.2.1 Data Files A data file is a conglomeration of objects, which includes Tables, keys, views, stored procedures and others. All these objects are necessary for the efficient operation of the database. 2.8.2.2 Tables The data in a relational database resides in Tables. These are the building blocks of the […]
Keys and Relationships – Introduction to Databases
2.8.2.5 Keys and Relationships A primary key is created for each Table in the database to efficiently access records and to ensure entity integrity. This implies that each record in a Table is unique in some way. Therefore, no two records can have the same primary key. The primary key […]
Transaction Log Files – Introduction to Databases
2.8.2.7 Transaction Log Files A transaction is a logical group of SQL statements that carry out a unit of work. Client server databases use log files to keep track of transactions that are applied to the database. For example, before an update is applied to a database, the database server […]