MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6

Spatial values, or geometries, have the properties described at Section 11.5.2.2, “Geometry Class”. The following discussion lists general spatial function argument-handling characteristics. Specific functions or groups of functions may have additional argument-handling characteristics, as discussed in the sections where those function descriptions occur.

Spatial functions are defined only for valid geometry values.

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
2^{32}−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.

Geometry values produced by any spatial function inherit the SRID of the geometry arguments.

Spatial functions that take multiple geometry arguments require
those arguments to have the same SRID value (that is, same value
in the lower 32 bits). 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.

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.