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Introduction
Functions returning geometries
Collections
Summary
15. Summary

## Instruction

Awesome! We learned quite a lot of valuable information in this section, so it's time to wrap things up!

• ST_Centroid(geom) and ST_PointOnSurface(geom) return points that represent the center of a geometry. The latter function guarantees that the point will lie on the inside the geometry but is slower.
• ST_Buffer(geom, distance) returns a bounding region called a "buffer" for a given geometry whose boundaries are 'distance' away from the geometry's boundaries.
• ST_Intersection(geom1, geom2) returns the shared part of two geometries.
• ST_Union(geom1, geom2, ...) returns the sum of geometries. ST_Union(geo_column) is an aggregate variant that works like SUM(...).
• Collections are sets of geometries stored together. These geometries need not all be of the same subtype.
• ST_GeometryN(geometry, n) returns the nth geometry of the given collection.
• ST_NumGeometries(geometry) returns the number of elements in the given collection.
• ST_Area and ST_Length can also be used with collections.

Alright, how about a short quiz now?

## Exercise

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