MySQL 8.3 Release Notes
For geometry values, MySQL distinguishes between the concepts of syntactically well-formed and geometrically valid.
A geometry is syntactically well-formed if it satisfies conditions such as those in this (nonexhaustive) list:
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Linestrings have at least two points
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Polygons have at least one ring
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Polygon rings are closed (first and last points the same)
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Polygon rings have at least 4 points (minimum polygon is a triangle with first and last points the same)
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Collections are not empty (except
GeometryCollection)
A geometry is geometrically valid if it is syntactically well-formed and satisfies conditions such as those in this (nonexhaustive) list:
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Polygons are not self-intersecting
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Polygon interior rings are inside the exterior ring
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Multipolygons do not have overlapping polygons
Spatial functions fail if a geometry is not syntactically well-formed. Spatial import functions that parse WKT or WKB values raise an error for attempts to create a geometry that is not syntactically well-formed. Syntactic well-formedness is also checked for attempts to store geometries into tables.
It is permitted to insert, select, and update geometrically invalid geometries, but they must be syntactically well-formed. Due to the computational expense, MySQL does not check explicitly for geometric validity. Spatial computations may detect some cases of invalid geometries and raise an error, but they may also return an undefined result without detecting the invalidity. Applications that require geometrically-valid geometries should check them using the ST_IsValid() function.