Difference
DIFFERENCE(g1 Geometry, g2 Geometry)
Alias: ST_DIFFERENCE
Returns the part of g1 that does not share any points with g2. g1 and g2 should have the same dimension.
Result type: geometry
Examples:
g2 shares every one of its points with g1; g1 also has some unique points:
SELECT ASTEXT(DIFFERENCE(POLYFROMTEXT('POLYGON((0 0, 10 0, 10 10, 0 10, 0 0))'), POLYFROMTEXT('POLYGON((2 2, 2 4, 4 4, 4 2, 2 2))')));
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col1
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POLYGON ((0 0, 0 10, 10 10, 10 0, 0 0), (2 2, 4 2, 4 4, 2 4, 2 2))
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g1 does not share any points with g2:
SELECT ASTEXT(DIFFERENCE(POLYFROMTEXT('POLYGON((0 0, 4 0, 4 10, 0 10, 0 0))'), POLYFROMTEXT('POLYGON((6 0, 10 0, 10 10, 6 10, 6 0))')));
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col1
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POLYGON ((0 0, 0 10, 4 10, 4 0, 0 0))
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g1 shares every one of its points with g2:
SELECT ASTEXT(DIFFERENCE(POLYFROMTEXT('POLYGON((2 2, 2 8, 8 8, 8 2, 2 2))'), POLYFROMTEXT('POLYGON((0 0, 10 0, 10 10, 0 10, 0 0))')));
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col1
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GEOMETRYCOLLECTION EMPTY
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