1. A computer program product for joining operations on document-oriented databases, the computer program product comprising: one or more non-transitory computer readable storage media and program instructions stored on the one or more non-transitory computer readable storage media, the program instructions comprising: program instructions to, receive a plurality of database identifiers for joining each of a core database and one or more target databases; program instructions to, identify the core database and the one or more target databases based on the plurality of database identifiers, respectively; program instructions to, receive one or more common attributes associated with the core database; program instructions to, receive one or more results attributes for generating combined join results; program instructions to, search for the one or more common attributes and the one or more results attributes in the core database and the one or more target databases, wherein searching comprises: program instructions to, determine a replacement attribute for at least one of the one or more common attributes based on one or more join substitution data stores, and the searching further comprises: one of one or more string search, one or more character search, one or more expansive search wherein the one or more expansive search comprises searching based on the replacement attribute, the replacement attribute determined based on one or more expansion terms data stores, and one or more nested search traversing a core database schema levels and one or more target databases schema levels, using the one or more string search and at least one of the one or more character search and the one or more expansive search; program instructions to, update the one or more join substitution data stores by receiving input to resolve one or more common attribute join conflicts; program instructions to, update the one or more expansion terms data store based on user input; program instructions to, identify the one or more common attribute join conflicts wherein the one or more common attribute join conflicts comprise multiple combinations to join attributes; program instructions to, create one or more choices for each of the one or more common attribute join conflicts based on searching for one or more matching join conflict resolutions in the one or more join substitution data stores based on each of the one or more common attribute join conflicts and one or more matching join conflict resolutions; program instructions to, output the one or more choices for one or more choice selections; program instructions to, receive the one or more choice selections as a second one or more common attribute join conflict resolutions, to resolve the one or more common attribute join conflicts; program instructions to, store the second one or more common attribute join conflict resolutions in the one or more join substitution data stores; and program instructions to, output one or more virtual tables wherein the one or more virtual tables comprises the combined join results based on the one or more common attributes and the one or more results attributes from the core database and the one or more target databases.