Question

给定一个包含几何数据的多边形表并使用ST_INTERESECTS（），我们如何返回相交的多边形，同时防止我们的结果变得多余？

要清楚的是，如果我们返回多边形A与多边形B相交，那么多余的也会返回多边形B与多边形A相交的行。

示例：

SELECT table1.name, table2.name 
FROM tl AS table1, tl AS table2
WHERE table1.name <> table2.name  // prevent returning A intersects A
AND ST_INTERSECTS( table1.coords, table2.coords) 
ORDER BY table1.name asc;

返回

hi hello  
peanut butter   
hello hi  
butter peanut

我想要

hi hello  
peanut butter

Answer 1

您可以先确定始终首先显示较小的值，然后使用<而不是<>

SELECT table1.name, table2.name 
FROM tl AS table1, tl AS table2
WHERE table1.name < table2.name -- The aforementioned assumption
AND ST_INTERSECTS( table1.coords, table2.coords) 
ORDER BY table1.name asc;

Answer 2

另一个选择是对结果应用DISTINCT：

SELECT DISTINCT least(table1.name, table2.name) as name1, 
                greatest(table1.name, table2.name) as name2
FROM tl AS table1, tl AS table2
WHERE table1.name <> table2.name  -- prevent returning A intersects A
AND ST_INTERSECTS( table1.coords, table2.coords) 
ORDER BY table1.name asc;

PostgreSQL如何返回元组{a，b}而不是元组{b，a}？

2 个答案: