Examples  

Legal:   r0(X,Y,Z) & r1(Y,U) & r2(Z,R) & r3(U,R) & X = x0 & R ³ medium

assuming:

• object relation is r₀,
•  nominal or tree structred, R ordered,
• foreign links is F = {r_o[2]-> r₁[1], r₀[3] ->  r₂[1],  r₁[2]->   r₃[1],  r₂[2]->  r₃[2]}.

 

Illigal:   r0(X,Y,Z) & r3(U,R)    [not linked]   r0(X,Y,Z) & r3(X,R)    [violates for. links]   r1(Y,U) & r2(Z,R) & r3(U,R)    [ no obj. relation]

   

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