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MultiPoint は、Point 要素から構成される幾何図形コレクションです。点の接続や順序付けは一切行われません。
MultiPoint の例
世界地図で、
MultiPointは一連の小さな島々を表すことができます。市内地図で、
MultiPointはチケットオフィスの系列店を表すことができます。
MultiPoint のプロパティー
MultiPointは 0 次元の幾何図形です。この 2 つの
Pointの値 (座標値) が等しくない場合は、MultiPointは単純になります。MultiPointの境界は空セットになります。
User Comments
User comments in this section are, as the name implies, provided by MySQL users.
The MySQL documentation team is not responsible for, nor do they endorse, any of
the information provided here.
Although that sounds easy, it can be counter-intuitive. The problem is the precision of coordinates. For example, in our project ( http://dobrotvor.com/ ) we bumped into this due to the fact that the precision of geo coordinates incapsulates the zoom value of Google Maps points. It's not right just to compare (X1 == X2) AND (Y1 == Y2) since the results can be wrong in many cases.
To make it work right, it's better to use special function (couldn't find anything better built-in):
[code]
DELIMITER //
DROP FUNCTION IF EXISTS ArePointsEqual; //
CREATE FUNCTION ArePointsEqual(p1 POINT, p2 POINT) RETURNS BOOLEAN NO SQL DETERMINISTIC
BEGIN
RETURN IsZero(X(p1) - X(p2)) AND IsZero(Y(p1) - Y(p2));
END; //
DROP FUNCTION IF EXISTS IsZero; //
CREATE FUNCTION IsZero(n DOUBLE) RETURNS BOOLEAN NO SQL DETERMINISTIC
BEGIN
DECLARE epsilon DOUBLE DEFAULT 0.00000000001;
RETURN (ABS(n) <= epsilon);
END; //
[/code]
The trick is not to use 0 as "zero value", but to use a very small difference (called "epsilon") instead for comparison.
http://dev.mysql.com/doc/refman/5.6/en/problems-with-float.html