STContains

STContains is a spatial predicate used to determine whether one geometry contains another. It is implemented in various spatial libraries and database systems, including PostGIS and Microsoft SQL Server, as a boolean test that checks containment relations between two geometry objects.

Semantics of STContains generally state that geometry A contains geometry B if every point of B lies inside A or on A’s boundary. In other words, no points of B lie outside A. The exact treatment of boundary points and empty geometries can vary slightly between implementations, but the core idea is that B must be entirely within A or on its boundary for the predicate to be true.

Common implementations

- PostGIS (SQL standard): ST_Contains(geomA, geomB) returns true when B is entirely inside A, including points on A’s boundary.

- SQL Server: A.STContains(B) returns true if B is contained within A, with results dependent on the types geo/geometry and on how empty geometries are handled in a given version.

Usage

STContains is used in queries to filter results, perform spatial joins, or drive analytics that depend on containment relationships. For example, in PostGIS you might write:

SELECT ST_Contains(building_geom, parcel_geom) FROM properties;

In SQL Server:

SELECT parcelTable.* FROM parcels AS p JOIN buildings AS b ON p.geom.STContains(b.geom) = 1;

Considerations

- Coordinate systems should be compatible; mismatched SRIDs typically require transformation.

- The predicate often participates in spatial indexing to improve performance on large datasets.

- STContains can be used with both planar geometry and geographic types, though semantics differ (euclidean vs. geodesic containment) and may affect performance and accuracy.