Structure of relational databases

A relational database consists of a collection of tables, each of which is assigned a unique name. In general, a row in a table represents a relationship among a set of values.

• In the relational model, the term relation is used to refer to a table, whilst the term tuple is used to refer to a row.
• Similarly, the term attribute refers to a column of a table.

  • Denoted as $A_1,\cdots ,A_n$.

    

• A relation schema consists of a list of attributes and their corresponding domains.
  • Denoted as $R=(A_1,\cdots ,A_n)$.
• The term relation instance refers to a specific instance of a relation, i.e., containing a specific set of rows.
  • Denoted as $r(R)$.
• The domain of the attribute refers to the set of allowed values for each attribute.
  • Attribute values are normally required to be atomic, i.e., elements of the domain are considered to be indivisible units.
  • The special value null is a member of every domain, indicating that the value is unknown.
• Relations are unordered, like sets in mathematics.