PostgreSQL如何返回元組{a，b}但不是元組{b，a}？

Question

給定一個包含幾何數據和使用ST_INTERESECTS（）的多邊形表，如何返回相交的多邊形，同時防止我們的結果變得多餘？

需要明確的是，如果我們返回多邊形A相交多邊形B則是多餘的也返回行多邊形B相交多邊形A.

實施例：

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

你只需決定你總是首先提出的較小值，並且使用<而不只是<>

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

另一種選擇的是應用DISTINCT的結果：

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;