Conjunctive Query Homomorphism

def: a homomorphism of a Conjunctive Query $Q_2$ to a Conjunctive Query $Q_1$ is

  • a function $h$ that maps each variable in $Q_2$ to either
    • a variable from $Q_1$ or
    • a constant form $Q_1$
  • s.t.
    • $h(\text{head}_2) = \text{head}_1$
    • $h(\text{body}_2) \subseteq \text{body}_1$
    • i.e. the values returned by queries are always the same, and body of one query is a subset of the other's query body


Example 1


  • relation $R(A, B)$
  • 3 queries
    • $Q_0(x) \leftarrow R(x, 33)$ ($Q_0 \equiv \pi_A \sigma_{B = 33}(R)$)
    • $Q_1(x) \leftarrow R(x, x)$ ($Q_1 \equiv \pi_A \sigma_{A = B}(R)$)
    • $Q_2(x) \leftarrow R(x, y)$ ($Q_0 \equiv \pi_A (R)$)

We can see that

  • $Q_0 \subseteq Q_2$ (obvious from the RA expressions: $Q_0$ just restricts $Q_2$)
  • and $Q_1 \subseteq Q_2$ (same reasoning)


  • $x \mapsto x, y \mapsto 33$ is a homomorphism of $Q_2$ to $Q_0$
  • $x \mapsto x, y \mapsto x$ is a homomorphism of $Q_2$ to $Q_0$
  • there is no homomorphism from $Q_0$ to $Q_2$ or from $Q_0$ to $Q_1$:
    • there's a constant in $Q_0$ that occurs neither in $Q_1$ nor in $Q_2$
    • (i.e. for any $h$ there's no atom of the form $R(h(x), 33)$ in the body of $Q_1$ or $Q_2$)
  • there is no homomorphism from $Q_1$ to $Q_2$
    • for any $h(x), h(y)$, there is no atom of the form $R(h(x), h(y))$ in the body of $Q_2$

Example 2

Example from before:

  • $A(x, y) \leftarrow R(x, w), G(w, z), R(z, y)$
  • $B(x, w) \leftarrow R(x, w), G(w, w), R(w, y)$

There is a homomorphism $h$ from $A$ to $B$

  • $h: x \mapsto x, y \mapsto y, w \mapsto w, z \mapsto w$

But there is no homomorphism $h$ from $B$ to $A$

  • such $h$ would have to map $G(w, w)$ to $G(w, z)$
  • which would imply $w \mapsto w$ and w $\mapsto z$ at the same time
  • but it's not possible since $h$ must be a function

Example 3

Given two queries

  • $C_1(x) \leftarrow R(x, y), R(y, z), R(z, w)$
  • $C_2(x) \leftarrow R(x, y), R(y, x)$

There's a homomorphism from $C_1$ to $C_2$:

  • $h: x \mapsto x, y \mapsto y, z \mapsto x, w \mapsto y$
  • i.e.
    • for head $x \mapsto x$
    • for the 1st atom of $C_1$: $y \mapsto y$ (maps to 1st atom of $C_2$)
    • for the 2nd atom of $C_1$: $z \mapsto x$ (maps to 2nd atom of $C_2$)
    • for the 3rd atom of $C_1$: $w \mapsto y$ (maps again to 1st atom of $C_2$)

But there's no homomorphism from $C_2$ to $C_1$

  • suppose it existed
  • it would assign $x \mapsto x$ to map head of $C_2$ to head of $C_1$
  • in this case $R(h(x), h(y)) = R(x, h(y))$ must occur in the body of $C_1$
    • $\to$ the 1st atom of $C_2$ would be mapped to 1st atom of $C_1$
    • $\to$ $h$ must map $y \mapsto y$
  • analogously, 2nd atom of $C_2$ must be mapped to 2nd atom of $C_1$
    • $h$ must map $x \mapsto z$
    • but it cannot because we have already established $x \mapsto x$
  • since $h$ must be a function, there is no homomorphism from $C_2$ to $C_1$

See Also


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