45. The computing system of claim 44 , wherein a first collection of nodes associated with the first mapping rule includes: a first node representing a first relational expression including a relational algebra operation that references the first source entity and the second source entity, a second node linked to the first node representing a relational expression including the first source entity, and a third node linked to the first node representing a relational expression including the second source entity.
45. The computing system of claim 44 , wherein a first collection of nodes associated with the first mapping rule includes: a first node representing a first relational expression including a relational algebra operation that references the first source entity and the second source entity, a second node linked to the first node representing a relational expression including the first source entity, and a third node linked to the first node representing a relational expression including the second source entity. 49. The computing system of claim 45 , wherein the relational algebra operation is an aggregation operation.