sql - 通过自左连接传递聚合函数列

Question

我想在列上调用一次聚合函数(成本)，并通过自左连接(用于创建行的排列或组合)将其级联。这样我认为成本是 O(n) 并在连接 O(row^joins) 之后调用它。从这样的事情开始:

id | cost 
----------
1  |  5   
2  |  10  

我想做类似下面的事情。我可以通过选择一个临时表并加入它来做类似的事情，但是我想避免使用临时表...

CREATE TEMP TABLE tmp_750309_plans AS (SELECT *, cost(id) as cost  FROM plans WHERE plans.id IN (1,2,...));

SELECT * FROM tmp_750309_plans AS t1 LEFT JOIN tmp_750309_plans AS t2 ON ...

我更愿意做这样的事情:

SELECT id, cost(id) as cost FROM plans AS t1
LEFT JOIN t1 AS t2 
ON t1.id != t2.id
AND ...

要得到这样的东西:

id | cost | id | cost |
-----------------------
1  |  5   |NULL| NULL |
2  |  10  |3   | 15   |

如有任何帮助，我们将不胜感激。

Answer 1

无需创建临时表，只需将您的 SELECT 设为派生表即可:

SELECT * 
FROM (
  SELECT *, cost(id) as cost  
  FROM plans 
  WHERE plans.id IN (1,2,...)
) tmp AS t1 LEFT JOIN tmp AS t2 ON ...

或具有通用表表达式的替代解决方案(适用于 PostgreSQL 8.4 及更高版本)

with tmp as (
  SELECT *, cost(id) as cost  
  FROM plans 
  WHERE plans.id IN (1,2,...)
)
SELECT *
FROM tmp as T1
  LEFT JOIN tmp AS t2 ON ...