11 - Querying a set

[bbox:-25.38653, 130.99883, -25.31478, 131.08938]; //find all nodes with a name, storing results in a set called "named_nodes" node["name"]->.named_nodes; //extract from the set "named_nodes", just those with node.named_nodes["natural"="cave_entrance"]->.caves; .caves out body;

This query demonstrates syntax for querying and serialising the contents of a named set.

Rather than work with the default set, this query only uses named sets. This illustrates how every statement works with data from an input set and stores its results in an output set.

First we find nodes that have a name and store them in a set of results called "named_nodes".

Set names can include letters, digits and the underscore character. But names must not start with a digit.

Secondly, we then query within that first set of results, filtering to just those nodes that are tagged as a cave entrance.

For node queries we append the name of the set to the keyword using a dotted syntax:

node.named_nodes

This means the statement will use that set rather than the OSM database.

So node.named_nodes means "Query nodes found in the set 'named_nodes'". The nodes are then filtered based on the value of their natural tag.

The results of that query are written to another named set. This time called caves.

Finally, we output the contents of the caves set.

This query is a little artificial as we can achieve the same thing by querying the database directly. But in more complex queries you may be working with multiple sets of results that you want to filter, explore and output in different ways.

We can also output results from different sets through repeated uses of the out keyword that reference different sets.

Every OverpassQL query command can work with named sets as well as the default "_" set. They can also store their results in named sets. This provides a great deal of flexibility.

Source File	11-query-a-set.osm
Authors	Leigh Dodds