You can look for this:
 
let
    Source = Table.NestedJoin(#"mysqldb support_request", {"id"}, #"mysqldb support_request_tag"[[#"support_request_id"], [#"tag_name"]], {"support_request_id"}, "tag_name_csv", JoinKind.LeftOuter),
    #"Transformed tags" = Table.TransformColumns(Source, {{"tag_name_csv", each Text.Combine(_[#"tag_name"], ", "), type text}})
in
    #"Transformed tags"