select res.id as 'res.id', res.name as 'res.name', tag.name as 'tag.name'
from Res res, ResTags rt, Tags tag
where res.id *= rt.resrow and rt.tagid *= tag.id

res.id | res.name | tag.name
0 | result0 | null
0 | result0 | tagA
1 | result1 | tagA
1 | result1 | tagB
2 | result2 | tagA
2 | result2 | tagC

{
 "mappings": {
  "res": {
   "properties": {
    "id": { "type": "long"},
    "name": { "type": "string" },
    "tags": {
      "type": "nested",
      "properties": { "tagname": { "type": "string" }}
    }
   }
 }
}

input {
 jdbc {
  jdbc_driver_library => "jtds-1.3.1.jar"
  jdbc_driver_class => "Java::net.sourceforge.jtds.jdbc.Driver"
  jdbc_connection_string => "jdbc:jtds:sybase://hostname.com:1234/schema"
  jdbc_user => "george"
  jdbc_password => "monkey"
  jdbc_fetch_size => 100
  statement_filepath => "/home/george/sql"
 }
}
output {
 elasticsearch {
  action => "update"
  index => "myres"
  document_type => "res"
  document_id => "%{res.id}"
  script_lang => "groovy"
  hosts => [ "my.other.host.com:5921" ]
  upsert => ' {
    "id" : %{res.id},
    "name" : "%{res.name}",
    "tags" :[{ "tagname": "%{tag.name}" }]
  }'
  script => '
    if (ctx._source.res.tags.containsValue(null)) {
      // if null has been added replace it with actual value
      cts._source.res.tags = [{"tagname": "%{tag.name}" }];
    else {
      // if you find the tag, then do nothing
      if (ctx._source.res.tags.containsValue("%{tag.name}")) {}
      else {
        // if the value you try to add is not null
        if (%{tag.name} != null)
           // add it as a new object into the tag array
           ctx._source.res.tags += {"tagname": "%{tag.name}"};
        }
     }
   '
 }
}

{
 "hits": {
   "total": 3,
   "max_score": 1,
   "hits": [ {
     "_index": "myres",
     "_type": "res",
     "_id": 0,
     "_score": 1,
     "_source": {
       "res": {
         "id":0,
         "name": "result0",
         "tags": [{"tagname": "tagA"}],
         "@version": "2",
         "@timestamp": "2016-xx-yy..."
       }
      },{
     "_index": "myres",
     "_type": "res",
     "_id": 1,
     "_score": 1,
     "_source": {
       "res": {
         "id":1,
         "name": "result1",
         "tags": [{"tagname": "tagA"},{"tagname": "tagB"}],
         "@version": "2",
         "@timestamp": "2016-xx-yy..."
       }
      }{
     "_index": "myres",
     "_type": "res",
     "_id": 2,
     "_score": 1,
     "_source": {
       "res": {
         "id":2,
         "name": "result2",
         "tags": [{"tagname": "tagA"},{"tagname": "tagC"],
         "@version": "2",
         "@timestamp": "2016-xx-yy..."
       }
      }
     }
...

[400] {"error":"ActionRequestValidationException[Validation Failed:
1: script or doc is missing;
2: script or doc is missing;
3: script or doc is missing;],"status":400]} {:class=> ... bla bla ...}