ES提供的聚合接口相当灵活和丰富，值得系统的学习一下。学习方式想了下，应该跟SQL结合起来，SQL相对来说比较直观，ES官网对Aggregation描述的展开也是从SQL开始的，所以通过设计不同的SQL 聚合查询场景，分别看下ES中用Aggregation API和Java API是怎么实现的。

测试数据源于官网

POST /cars/transactions/_bulk
{ "index": {}}
{ "price" : 10000, "color" : "red", "make" : "honda", "sold" : "2014-10-28" }
{ "index": {}}
{ "price" : 20000, "color" : "red", "make" : "honda", "sold" : "2014-11-05" }
{ "index": {}}
{ "price" : 30000, "color" : "green", "make" : "ford", "sold" : "2014-05-18" }
{ "index": {}}
{ "price" : 15000, "color" : "blue", "make" : "toyota", "sold" : "2014-07-02" }
{ "index": {}}
{ "price" : 12000, "color" : "green", "make" : "toyota", "sold" : "2014-08-19" }
{ "index": {}}
{ "price" : 20000, "color" : "red", "make" : "honda", "sold" : "2014-11-05" }
{ "index": {}}
{ "price" : 80000, "color" : "red", "make" : "bmw", "sold" : "2014-01-01" }
{ "index": {}}
{ "price" : 25000, "color" : "blue", "make" : "ford", "sold" : "2014-02-12" }

测试数据是汽车的销售信息，这次实现的SQL是按颜色groupby，查询每种颜色的销售数目。

SELECT COUNT(color)  FROM table GROUP BY color

Aggregation API:

GET /cars/transactions/_search
{
    "size" : 0,
    "aggs" : { 
        "popular_colors" : { 
            "terms" : { 
              "field" : "color"
            }
        }
    }
}

java API：

 /**
  * test sql: SELECT COUNT(color)  FROM table GROUP BY color
  */
  def aggTest1(): Unit = {
    import scala.collection.JavaConverters._
    val agg = AggregationBuilders
      .terms("color")
      .field("color")
      .order(Terms.Order.count(false))
    val searchRes = queryBuilder.setQuery(boolQueryBuilder).addAggregation(agg).execute().actionGet()
    searchRes.getAggregations.get("color").asInstanceOf[Terms].getBuckets.asScala.foreach{ bucket =>
      println(s"key: ${bucket.getKeyAsString}, value ${bucket.getDocCount}")
    }
  }