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DOUBLE ST_ZMin(GEOMETRY geom);
Returns the minimum z-value of geom.
geom
SELECT ST_ZMin('LINESTRING(1 2 3, 4 5 6)'); -- Answer: 3.0
ST_XMin, ST_XMax, ST_YMax, ST_YMin, ST_ZMax
ST_XMin
ST_XMax
ST_YMax
ST_YMin
ST_ZMax
Source code