I. Datalog

Datalog is a declarative logic programming language that is a subset of Prolog.

• It emerged as a database language (mid-1980s)*
• Now it has a variety of applications
  • Program analysis
  • Declarative networking
  • Big data
  • Cloud computing

Features

$$ \textrm{Datalog} = \textrm{\red{Data}} + \textrm{\red {Log}ic} $$

• No side-effects
• No control flows
• No functions
• Not Turing-complete

Data - Predicates

In Datalog, a predicate (relation) is a set of statements

Essentially, a predicate is a table of data

A fact asserts that a particular tuple (a row) belongs to a relation (a table), i.e., it represents a predicate being true for a particular combination of values

Atoms

Atoms are basic elements of Datalog, which represent predicates of the form

Terms:

• Variables: stand for any values
• Constants

Examples

• Age(person,age)
• Age(“Xiaoming”,18)

$P(X_1,X_2,…,X_n)$ is called relational atom

• evaluates to true when predicate P contains the tuple described by $X_1,X_2,…,X_n$