Search notes:

Overpass API: out statement

rel
  [boundary=administrative]
  [name    =Pfungen       ]
;

//  >;

out        //     <relation id="1682188">
//  meta   //     <relation id="1682188" version="5" timestamp="2016-12-07T02:43:55Z" changeset="44224658" uid="339581" user="nyuriks">

//  bb     //     Bounding box for each selected element?
//         //     <relation id="1682188">
//         //        <bounds minlat="47.4974379" minlon="8.6176417" maxlat="47.5218535" maxlon="8.6611408"/>

//  center //     <relation id="1682188">
           //        <center lat="47.5096457" lon="8.6393912"/>

//  geom   //     include coordinates in <nd …> tags.
           //     Seems to be required to draw ways on the map.

//  skel   //     Don't emit <tag k="…" v="…"> nodes.

//  ids    //     Output IDs only

//  tags   //     Output IDs and tags

    count  //     get number of nodes, ways, relations and their total.

//  qt     //     Sort by quadtile index
//                This is roughly geographical and significantly faster than order by ids.
//                See also https://gis.stackexchange.com/a/187754:

//  body   //     ?

//  popup  //     Invalid parameter «popup»
;

Named set

The content of a set can be outputted by prepending out statement with the name of the set (and the usual dot): 
(
   make t1
       num = 42,
       txt = 'hello world';
  
   make t2
       num = 99,
       txt = 'ninety-nine';
  
) -> .x;

.x out;

bb

out bb reports the bounding box of relations in a <bounds> tag with minlat, minlon, maxlat and maxlon attributes, for example: 
…
  <relation id="1682188">
    <bounds minlat="47.4974379" minlon="8.6176417" maxlat="47.5218535" maxlon="8.6611408"/>
    …
  </relation>
…
See also the bbox setting.

count

out count; prints the total number of elements in the input set, broken down to type (i. e. nodes, ways, relations, areas).
It cannot be combined with another out specifier.
The following query counts the number of ways and nodes in relation 5305784: 
relation(5305784);
>>;

out count;
The output is something like 
<count id="0">
  <tag k="nodes" v="2106"/>
  <tag k="ways" v="2"/>
  <tag k="relations" v="1"/>
  <tag k="total" v="2109"/>
</count>

Count the number of amenity=drinking_water nodes, ways and relations in each canton of Switzerland

[out:csv(
   ::count,
   ::"count:nodes",
   ::"count:ways",
   ::"count:relations"
)];

area['ISO3166-2' ~ '^CH-'];

foreach -> .kanton(

   nwr
     (area.kanton)
     [amenity = drinking_water];

   out count;
);
Almost the same thing, but with foreach.

Special field names

Special field names (starting with two colons (::…)) make it easy to export OSM data as CSV: 
[out:csv (

 //
 //   Tag names:
 //
      amenity          ,
      name             ,
     'addr:street'     ,
     'addr:housenumber', 
 //
 //   Special field names:
 //
    ::id         , //  OSM object ID
    ::type       , //  Object type (node, way or relation)
    ::otype      , //  Object type as numeric value
    ::lat        , //  Latitude + Longitude (available for nodes
    ::lon        , //      or ways with 'out center')
 //
 //   Field names available with 'out meta'
 //
    ::version    , //  OSM object's version number
    ::timestamp  , //  Timestamp of last modificatio
    ::changeset  , //  Changeset in which the object was changed
    ::uid        , //  User id of mapper
    ::user       ; //  User name of mapper
 //
 //  Header and separator
 //
     true   ;
     " | "
)];

{{geocodeArea: Pfungen }} -> .pfungen;

nwr[amenity](area.pfungen);

out meta;
Some field names are available for the out count statement 
[out:csv (
 //
 //   Special field names (for 'out count')
 //
    ::count            , //    Returns total number of objects (nodes, ways, relations and areas) in inputset
    ::"count:nodes"    , //    Returns number of nodes in inputset
    ::"count:ways"     , //    Returns number of ways in inputset
    ::"count:relations", //    Returns number of relations in inputset
    ::"count:areas"    ; //    Returns number of areas in inputset
 //
 //  Header and separator
 //
     true   ;
     " | "
)];

{{geocodeArea: Pfungen }} -> .pfungen;

nwr(area.pfungen);

out count;

ids

Print the ids of all nodes whose name is Matterhorn and are also peaks. It turns out that there are more than just one Matterhorn… 
node
   [name    = Matterhorn]
   [natural = peak      ];

out ids;

tags

relation[admin_level=2];

out tags;
The previous query only emits relations and their tags, without also listing the relations' members: 
<?xml version="1.0" encoding="UTF-8"?>
<osm version="0.6" generator="Overpass API 0.7.59.2 0994154d">
<note>The data included in this document is from www.openstreetmap.org. The data is made available under ODbL.</note>
<meta osm_base="2023-03-11T11:04:42Z"/>

  <relation id="9407">
    <tag k="ISO3166-1" v="AD"/>
    ….
    <tag k="name" v="Andorra"/>
  </relation>
  <relation id="11980">
    <tag k="ISO3166-1" v="FR"/>
    …
    <tag k="name" v="France (terres)"/>
    …

See also

The out setting is unrelated to the out statement.
Overpass API: statements

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