//  Datalog program.
//  Facts:
likePizza('bun');
likePizza('john');
likePizza('mary');
hasPizza('bun');
hasPizza('john');

//  Rules: if-then statement.
//  head :- body (p1, p2, p3,...); ,: and.
happy(X) :- hasPizza(X), likePizza(X);

happy('bun');   
true
happy('susan');
false
happy(X);  //  X variable.
X = bun
X = john

boss(X,Y) :- supervise(X,Y);  R1
boss(X,Y) :- supervise(X,Z), boss(Z,Y); R2

supervise('joe','mary'); 
supervise('mary','peter'); 
supervise('mary','paul'); 
supervise('paul','stan'); 
supervise('paul','lee'); 

Queries 

boss('joe','mary');
true: R1

boss('mary','joe'); 
boss('mary','stan');
true: R2 (bind X; 'mary', Y: 'stan') 
     Look for supervise('mary', Z), 
           found supervise('mary','paul'): Z: 'paul'
           Look for boss('paul', 'stan')
           R1: Look for supervise('paul','stan'): T
boss(X,'lee'); 
boss('joe',Y); 
boss(A,B);