DELIMITER $$
CREATE FUNCTION trueWithin(p POINT, poly POLYGON)
RETURNS INT(1) DETERMINISTIC
BEGIN
    DECLARE n INT DEFAULT 0;
    DECLARE nr INT DEFAULT 0;
    DECLARE pX DECIMAL(9,6);
    DECLARE pY DECIMAL(9,6);
    DECLARE ls LINESTRING;
    DECLARE poly1 POINT;
    DECLARE poly1X DECIMAL(9,6);
    DECLARE poly1Y DECIMAL(9,6);
    DECLARE poly2 POINT;
    DECLARE poly2X DECIMAL(9,6);
    DECLARE poly2Y DECIMAL(9,6);
    DECLARE i INT DEFAULT 0;
    DECLARE j INT DEFAULT 1;
    DECLARE result INT(1) DEFAULT 0;
    SET pX = X(p);
    SET pY = Y(p);
    SET ls = ExteriorRing(poly);
    SET poly2 = EndPoint(ls);
    SET poly2X = X(poly2);
    SET poly2Y = Y(poly2);
    SET n = NumPoints(ls);
    WHILE i<n DO
        SET poly1 = PointN(ls, (i+1));
        SET poly1X = X(poly1);
        SET poly1Y = Y(poly1);
        IF ( ( ( (poly1X <= pX) && (pX < poly2X) ) || ( (poly2X <= pX) && (pX < poly1X) ) ) && ( pY > (poly2Y - poly1Y) * (pX - poly1X) / (poly2X - poly1X) + poly1Y ) ) THEN
            SET result = !result;
        END IF;
        SET poly2X = poly1X;
        SET poly2Y = poly1Y;
        SET i = i + 1;
    END WHILE;
    SET nr = NumInteriorRings(poly);
    WHILE j<=nr DO
        SET ls = InteriorRingN(poly, j);
        SET poly2 = EndPoint(ls);
        SET poly2X = X(poly2);
        SET poly2Y = Y(poly2);
        SET n = NumPoints(ls);
        SET i = 0;
        WHILE i<n DO
            SET poly1 = PointN(ls, (i+1));
            SET poly1X = X(poly1);
            SET poly1Y = Y(poly1);
            IF ( ( ((poly1X <= pX) && (pX < poly2X)) || ((poly2X <= pX) && (pX < poly1X)) ) && ( pY > (poly2Y - poly1Y) * (pX - poly1X) / (poly2X - poly1X) + poly1Y ) ) THEN
                SET result = !result;
            END IF;
            SET poly2X = poly1X;
            SET poly2Y = poly1Y;
            SET i = i + 1;
        END WHILE;
        SET j = j + 1;
    END WHILE;
    RETURN result;
End$$
DELIMITER ;

/* usage: */
SELECT trueWithin(PointFromText('POINT(-94.584799 39.100973)'), (SELECT `shape` FROM `table` WHERE `id` = 'constraint')) AS result;