Exercises

Relational Model

• consider A1, A2,.., An a set of attributes (or columns, fields, data names, etc) and Di = Dom(Ai) U {?} the domain of possible values of attribute Ai, where “?” is denoted “undefined” (null) value
• “undefined” value is used to specify if an attribute has a value assigned or it has no value
• this value does not belong to a specific data type and it could be used in expression together with other attributes values having different types (number, text, data, etc..)

Relations

• relation = table with columns and rows

• attributes = columns in a table, representing the data type

• tuples = rows in a table, representing a set of related values

  a table Students with columns ID, Name, and Major
  

• a relational database is a set of relations

• a database schema is the set of schemas of the relations in the database

• a database instance (state) is the set of instances of the relations in the database

• a relational database has 3 parts:

  • data (relations content) and its description
  • integrity constraints (to preserve database consistency)
  • data management operators

• a field is a data structure for a single piece of data, fields are organized into records, which contain all the information within the table relevant to a specific entity

• a field or a set of fields is a key for a relation; if there are not, 2 tuples can have the same values for all fields and this is not true for any subset of the key

• a field is a data structure for a single piece of data, fields are organized into records, which contain all the information within the table relevant to a specific entity
• a field or a set of fields is a key for a relation. if there are not, 2 tuples can have the same values for all fields and this is not true for any subset of the key
• a relation may have more keys: one of them is chosen as a primary key, all the others being considered secondary keys ( or key candidates)