1. A non-transitory computer readable storage medium having instructions stored thereon that, when executed by a processor, causes the processor to generate one or more recommended items for a user, the generating comprising: receiving a target items set for the user, wherein the target items set comprises two or more target items; querying a database of a plurality of related items sets using the target items set, wherein each related items set comprises a plurality of related items, wherein the querying comprises determining a similarity between the target items set and the plurality of related items sets; in response to the querying and the similarity determination, retrieving one or more similar items sets from the plurality of related item sets, wherein each similar items set is retrieved when the similar items set has one or more items in common with the target items set; and performing a union of the similar items sets to generate the recommended items.