14.16.12 Spatial Aggregate Functions

MySQL supports aggregate functions that perform a calculation on a set of values. For general information about these functions, see Section 14.19.1, “Aggregate Function Descriptions”. This section describes the ST_Collect() spatial aggregate function.

ST_Collect() can be used as a window function, as signified in its syntax description by [over_clause], representing an optional OVER clause. over_clause is described in Section 14.20.2, “Window Function Concepts and Syntax”, which also includes other information about window function usage.

• ST_Collect([DISTINCT] g) [over_clause]

  Aggregates geometry values and returns a single geometry collection value. With the DISTINCT option, returns the aggregation of the distinct geometry arguments.

  As with other aggregate functions, GROUP BY may be used to group arguments into subsets. ST_Collect() returns an aggregate value for each subset.

  This function executes as a window function if over_clause is present. over_clause is as described in Section 14.20.2, “Window Function Concepts and Syntax”. In contrast to most aggregate functions that support windowing, ST_Collect() permits use of over_clause together with DISTINCT.

  ST_Collect() handles its arguments as follows:

  • NULL arguments are ignored.

  • If all arguments are NULL or the aggregate result is empty, the return value is NULL.

  • If any geometry argument is not a syntactically well-formed geometry, an ER_GIS_INVALID_DATA error occurs.

  • If any geometry argument is a syntactically well-formed geometry in an undefined spatial reference system (SRS), an ER_SRS_NOT_FOUND error occurs.

  • If there are multiple geometry arguments and those arguments are in the same SRS, the return value is in that SRS. If those arguments are not in the same SRS, an ER_GIS_DIFFERENT_SRIDS_AGGREGATION error occurs.

  • The result is the narrowest MultiXxx or GeometryCollection value possible, with the result type determined from the non-NULL geometry arguments as follows:

    • If all arguments are Point values, the result is a MultiPoint value.

    • If all arguments are LineString values, the result is a MultiLineString value.

    • If all arguments are Polygon values, the result is a MultiPolygon value.

    • Otherwise, the arguments are a mix of geometry types and the result is a GeometryCollection value.

  This example data set shows hypothetical products by year and location of manufacture:

  CREATE TABLE product (
    year INTEGER,
    product VARCHAR(256),
    location Geometry
  );
  
  INSERT INTO product
  (year,  product,     location) VALUES
  (2000, "Calculator", ST_GeomFromText('point(60 -24)',4326)),
  (2000, "Computer"  , ST_GeomFromText('point(28 -77)',4326)),
  (2000, "Abacus"    , ST_GeomFromText('point(28 -77)',4326)),
  (2000, "TV"        , ST_GeomFromText('point(38  60)',4326)),
  (2001, "Calculator", ST_GeomFromText('point(60 -24)',4326)),
  (2001, "Computer"  , ST_GeomFromText('point(28 -77)',4326));

  Some sample queries using ST_Collect() on the data set:

  mysql> SELECT ST_AsText(ST_Collect(location)) AS result
         FROM product;
  +------------------------------------------------------------------+
  | result                                                           |
  +------------------------------------------------------------------+
  | MULTIPOINT((60 -24),(28 -77),(28 -77),(38 60),(60 -24),(28 -77)) |
  +------------------------------------------------------------------+
  
  mysql> SELECT ST_AsText(ST_Collect(DISTINCT location)) AS result
         FROM product;
  +---------------------------------------+
  | result                                |
  +---------------------------------------+
  | MULTIPOINT((60 -24),(28 -77),(38 60)) |
  +---------------------------------------+
  
  mysql> SELECT year, ST_AsText(ST_Collect(location)) AS result
         FROM product GROUP BY year;
  +------+------------------------------------------------+
  | year | result                                         |
  +------+------------------------------------------------+
  | 2000 | MULTIPOINT((60 -24),(28 -77),(28 -77),(38 60)) |
  | 2001 | MULTIPOINT((60 -24),(28 -77))                  |
  +------+------------------------------------------------+
  
  mysql> SELECT year, ST_AsText(ST_Collect(DISTINCT location)) AS result
         FROM product GROUP BY year;
  +------+---------------------------------------+
  | year | result                                |
  +------+---------------------------------------+
  | 2000 | MULTIPOINT((60 -24),(28 -77),(38 60)) |
  | 2001 | MULTIPOINT((60 -24),(28 -77))         |
  +------+---------------------------------------+
  
  # selects nothing
  mysql> SELECT ST_Collect(location) AS result
         FROM product WHERE year = 1999;
  +--------+
  | result |
  +--------+
  | NULL   |
  +--------+
  
  mysql> SELECT ST_AsText(ST_Collect(location)
           OVER (ORDER BY year, product ROWS BETWEEN 1 PRECEDING AND CURRENT ROW))
           AS result
         FROM product;
  +-------------------------------+
  | result                        |
  +-------------------------------+
  | MULTIPOINT((28 -77))          |
  | MULTIPOINT((28 -77),(60 -24)) |
  | MULTIPOINT((60 -24),(28 -77)) |
  | MULTIPOINT((28 -77),(38 60))  |
  | MULTIPOINT((38 60),(60 -24))  |
  | MULTIPOINT((60 -24),(28 -77)) |
  +-------------------------------+