--- layout: doc_page 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 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": , "expression": , "outputType":

} ``` |property|description|required?| |--------|-----------|---------| |name|The name of the virtual column.|yes| |expression|An [expression](../misc/math-expr.html) 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|