druid/docs/querying/virtual-columns.md

id	title
virtual-columns	Virtual Columns

Virtual columns are queryable column "views" created from a set of columns during a query.

A virtual column can potentially draw from multiple underlying columns, although a virtual column always presents itself as a single column.

Virtual columns can be used as dimensions or as inputs to aggregators.

Each Apache Druid query can accept a list of virtual columns as a parameter. The following scan query is provided as an example:

{
 "queryType": "scan",
 "dataSource": "page_data",
 "columns":[],
 "virtualColumns": [
    {
      "type": "expression",
      "name": "fooPage",
      "expression": "concat('foo' + page)",
      "outputType": "STRING"
    },
    {
      "type": "expression",
      "name": "tripleWordCount",
      "expression": "wordCount * 3",
      "outputType": "LONG"
    }
  ],
 "intervals": [
   "2013-01-01/2019-01-02"
 ]
}

Virtual column types

Expression virtual column

The expression virtual column has the following syntax:

{
  "type": "expression",
  "name": <name of the virtual column>,
  "expression": <row expression>,
  "outputType": <output value type of expression>
}

property	description	required?
name	The name of the virtual column.	yes
expression	An expression that takes a row as input and outputs a value for the virtual column.	yes
outputType	The expression's output will be coerced to this type. Can be LONG, FLOAT, DOUBLE, or STRING.	no, default is FLOAT