The Relational Model

The Relational Model

1. An entity is something of importance to a user that needs to be represented in a database.(represents one theme)

2. A relation is a two-dimensional table that has specific characteristics. (like a matrix, consist of rows and columns)

   • Characteristics of a Relation: o Rows contain data about an entity o Columns contain data about attributes of the entity o Cells of the table hold a single value o All entries in a column are of the same kind o Each column has a unique name o The order of the rows and columns are unimportant o No two rows may be identical

3. Synonyms : Table = File = Relation Row = Record = Tuple Column = Field = Attribute

4. A key is one (or more) column(s) of a relation that is (are) used to identify a row.

   • o Nonunique Key : Data value may be shared among several rows
   • o Unique Key : Data value is unique for each row

5. A candidate key is called “candidate” because it is a candidate to become the primary key. A candidate key is a unique key.

6. A composite key is a key that contains two or more attributes. For a key to be unique, it must often become a composite key.

7. A primary key is a candidate key chosen to be the main key for the relation.

8. A surrogate key is a unique, numeric value that is added to a relation to serve as the primary key.A surrogate key is often used in place of a composite primary key.

9. To preserve relationships, you may need to create a foreign key.A foreign key is a primary key from one table placed into another table.The key is called a foreign key in the table that received the key

10. Referential integrity states that every value of a foreign key must match a value of an existing primary key.

11. Functional Dependency :a relationship between attributes in which one attribute (or group of attributes) determines the value of another attribute in the same table.

12. The attribute that we use as the starting point (the variable on the left side of the equation) is called a determinant.