PostgreSQL錯誤：子查詢必須只返回一列

Question

使用PostgreSQL-9.1和PostGIS 2.0.1時，在執行包含返回多列的子查詢的SELECT查詢時，出現錯誤subquery must return only one column。

如何修改查詢/子查詢以返回多個列？

查詢

SELECT l.id, l.lat, l.lng, l.geom, 
     (SELECT g.id, g.lat, g.lng, ST_Distance(l.geom, g.geom) 
     FROM stage.dogs as g 
     LIMIT 1) 

FROM stage.users As l 

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完整的查詢

SELECT l.id, l.lat, l.lng, l.geom, 
    g.id, g.lat, g.lng, ST_Distance(l.geom, g.geom) 
FROM stage.users As l 
CROSS JOIN (SELECT * 
    FROM stage.dogs as g 
    ORDER BY g.geom <-> l.geom 
    LIMIT 1) as g 

錯誤

ERROR: invalid reference to FROM-clause entry for table "l" 
SQL state: 42P01 
Hint: There is an entry for table "l", but it cannot be referenced from this part of the query. 

Answer 1

SELECT l.id, l.lat, l.lng, l.geom, 
     g.id, g.lat, g.lng, ST_Distance(l.geom, g.geom) 
FROM stage.users As l 
CROSS JOIN (SELECT * FROM stage.dogs LIMIT 1) as g 

這實際上是你所擁有的（假設stage.dogs）不是空的。不知道是否應該有users和dogs之間的相關性。

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要找到離用戶最近的狗，可以使用此查詢。標量子查詢查找狗的ID，並將其返回到表中以檢索其他列。

SELECT l.id, l.lat, l.lng, l.geom, 
     g.id, g.lat, g.lng, ST_Distance(l.geom, g.geom) 
FROM (
    SELECT l1.*, (SELECT g1.id 
        FROM stage.dogs as g 
        ORDER BY g.geom <-> l.geom 
        LIMIT 1) g_id 
    FROM stage.users As l1 
) l 
JOIN stage.dogs as g ON g.id = l.g_id; 

公平的警告，這不會是一個快速查詢。

---

在執行更慢的風險，請參見下面的查詢多個表

SELECT l.id, l.lat, l.lng, l.geom, 
     g.id, g.lat, g.lng, ST_Distance(l.geom, g.geom) dog_distance, 
     c.id, c.lat, c.lng, ST_Distance(l.geom, c.geom) cat_distance, 
     b.id, b.lat, b.lng, ST_Distance(l.geom, b.geom) bird_distance 
FROM (
    SELECT l1.*, (SELECT g1.id 
        FROM stage.dogs as g1 
        ORDER BY g1.geom <-> l.geom 
        LIMIT 1) dog_id, 
       (SELECT c1.id 
        FROM stage.cats as c1 
        ORDER BY c1.geom <-> l.geom 
        LIMIT 1) cat_id, 
       (SELECT b1.id 
        FROM stage.cats as b1 
        ORDER BY b1.geom <-> l.geom 
        LIMIT 1) bird_id 
    FROM stage.users As l1 
) l 
LEFT JOIN stage.dogs as g ON g.id = l.dog_id 
LEFT JOIN stage.dogs as c ON c.id = l.cat_id 
LEFT JOIN stage.dogs as b ON b.id = l.bird_id; 

Answer 2

這給你每個用戶一行與最接近的狗：上

SELECT DISTINCT ON (l.id) 
     l.id, l.lat, l.lng, l.geom 
     ,g.id, g.lat, g.lng, ST_Distance(l.geom, g.geom) 
FROM stage.users  l 
CROSS JOIN stage.dogs g 
ORDER BY l.id, (l.geom <-> g.geom) 

更多信息該技巧與DISTINCT ON在此有關的答案：

• Select first row in each GROUP BY group?

我想如果你有GiST index上g.geom，規劃可能是足夠聰明，只是從中挑選關閉項目。不確定，沒有測試。否則，這種CROSS JOIN會導致O（N²），並且性能可能會隨着更大的表格快速失控。

我引用POSTGIS手動here：

Index only kicks in if one of the geometries is a constant (not in a subquery/cte). e.g. 'SRID=3005;POINT(1011102 450541)'::geometry instead of a.geom

所以，你可能是出於運氣在這裏。

根據手冊，您可能需要訂購ST_Distance()才能獲得精確的排序順序，但是您不應該得到最遠的一個。這是沒有意義的。