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{{DISPLAYTITLE:Sub-Query Data Adapter}}
[[Category:AppBoard 2.6]]

The Sub-Query data adapter is used to perform queries and transformation against other data source entities. This does not appear in the adapter type chooser when adding a data source, instead select an existing data source and click the ''Create Sub-Query'' button.

Typical uses for the Sub-Query data adapter are when raw information needs to be summarized or transformed in a way to support a particular visualization:

* Grouping: group by one or more columns and calculate counts, or max/min values.
* Pivoting: transform row-oriented data into column-oriented data
* Sub-select: select a sub-set of columns
* Filtering: general operator and value based filtering to reduce the data set.
* Sorting: sort by one or more columns
* Limit Row Count: reduce the data set - returns the top most rows within the limit.

{{Note|Data Collections also provide some overlapping functionality, specifically the ability to sort, limit rows, and filtering.}}

[[File:appboard-2.4-sub-query-example.png|frame|Sub-Query example configuration.]]

=== Group-By Example ===

Given a raw list of items it's a fairly common task to be able to group these by some category in order to get min/max/counts for the groupings.

As an example, AppBoard ships with some sample data that includes airports around the world. We want to create a new data source that provides us with a count of airports by country, this may be useful to provide a pick-list of countries for filtering, or a pie-chart with the Top-10 Countries with the highest number of airports, etc...

The screenshot above shows the source as <tt>tutorial.Airport</tt>. Using the ''Advanced Transform'' option, and enabling a ''Transform'' we can then group by the <tt>Country</tt> column, and then perform an aggregation on some other column to give us a count. We could have added additional aggregation columns, such as the max and min on the <tt>Altitude</tt> column to give us some additional information about the groupings.

AppBoard 2.6 ships with some welcome content including a Sub-Query data source called <tt>airport.countries</tt> which does the same as this example. It's used to drive the example content to showcase the Tile Map widget.

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