Attribute

In relational databases, attributes are the describing characteristics or properties that define all items pertaining to a certain category applied to all cells of a column.

The rows, instead, are called tuples, and represent data sets applied to a single entity to uniquely identify each item. Attributes are, therefore, the characteristics of every individual tuple that help describe its unique properties.

Think of a table in a relational database as being analogous to an electronic spreadsheet. An attribute is simply one non-null cell in the spreadsheet, or the conjunction of a column and row.

It stores only one piece of data about the object represented by the table in which the attribute belongs. For example, the tuple can be an Invoice entity. The attributes of an invoice might be Price, Number, Date or Paid/unpaid.

Beyond the self-explanatory simple or single-valued attributes, there are several types of attributes available.

• Composite attribute: is an attribute composed of several other simple attributes. For example, the Address attribute of an Employee entity could consist of the Street, City, Postal code and Country attributes.
  
  
• Multivalued attribute: is an attribute where more than one description can be provided. For example, an Employee entity may have more than one Email ID attributes in the same cell.
  
  
• Key attribute or primary attribute: is an ID, key, letter or number that uniquely identifies that item. For example, it can be the number of a certain invoice (e.g. the individual ID of that invoice). A table that contains a single key attribute is considered a strong entity. However, a table might contain more than one key attribute if it’s derived from other tables.
  
  
• Derived attribute: as the name implies, these are derived from other attributes, either directly or through specific formula results. For example, the Age attribute of an Employee could be derived from the Date of Birth attribute. In other instances, a formula might calculate the VAT of a certain payment, so that whenever the cell with the attribute Payment is filled, the cell with the derived attribute VAT automatically calculates its value.