12.16.2 Argument Handling by Spatial Functions
Spatial values, or geometries, have the properties described in Section 220.127.116.11, “Geometry Class”. The following discussion lists general spatial function argument-handling characteristics. Specific functions or groups of functions may have additional or different argument-handling characteristics, as discussed in the sections where those function descriptions occur. Where that is true, those descriptions take precedence over the general discussion here.
Spatial functions are defined only for valid geometry values. See Section 11.4.4, “Geometry Well-Formedness and Validity”.
The spatial reference identifier (SRID) of a geometry identifies the coordinate space in which the geometry is defined. In MySQL, the SRID value is an integer associated with the geometry value. The maximum usable SRID value is 232-1. If a larger value is given, only the lower 32 bits are used.
In MySQL, all computations are done assuming SRID 0, regardless of the actual SRID value. SRID 0 represents an infinite flat Cartesian plane with no units assigned to its axes. In the future, computations may use the specified SRID values. To ensure SRID 0 behavior, create geometry values using SRID 0. SRID 0 is the default for new geometry values if no SRID is specified.
For computations on multiple geometry values, all values must have the same SRID or an error occurs. Thus, spatial functions that take multiple geometry arguments require those arguments to have the same SRID value. Assuming equal SRIDs, spatial functions do nothing with them after performing the equality check; geometry values are implicitly handled using Cartesian coordinates (SRID 0). If a spatial function returns
ER_GIS_DIFFERENT_SRIDS, it means that the geometry arguments did not all have the same SRID. You must modify them to have the same SRID.
Geometry values produced by any spatial function inherit the SRID of the geometry arguments.
The Open Geospatial Consortium guidelines require that input polygons already be closed, so unclosed polygons are rejected as invalid rather than being closed.
Empty geometry-collection handling is as follows: An empty WKT input geometry collection may be specified as
'GEOMETRYCOLLECTION()'. This is also the output WKT resulting from a spatial operation that produces an empty geometry collection.
During parsing of a nested geometry collection, the collection is flattened and its basic components are used in various GIS operations to compute results. This provides additional flexibility to users because it is unnecessary to be concerned about the uniqueness of geometry data. Nested geometry collections may be produced from nested GIS function calls without having to be explicitly flattened first.