1. A system for combining directed graphs, the system comprising: one or more processors; and a non-transitory computer readable medium storing a plurality of instructions, which when executed, cause the one or more processors to: identify, by a database system, a first directed graph having nodes connected by directed arcs and a second directed graph having nodes connected by directed arcs; create, by the database system, a combined directed graph having a corresponding node for: i) each node in the first directed graph lacking a corresponding node in the second directed graph, ii) each node in the second directed graph lacking a corresponding node in the first directed graph, and iii) each node in the first directed graph having a corresponding node in the second directed graph; create, by the database system, in the combined directed graph, a corresponding directed arc for: i) each directed arc in the first directed graph lacking a corresponding directed arc in the second directed graph, ii) each directed arc in the second directed graph lacking a corresponding directed arc in the first directed graph, and iii) each directed arc in the first directed graph having a corresponding directed arc in the second directed graph; and output, by the database system, a recommendation for a user to interact with a recommended object based on an object interaction and a conditional probability, in the combined directed graph, which corresponds to the recommended object and the object interaction.