通过自左联接传递聚合函数列

Question

我想在列上调用一次汇总函数（成本），然后通过自左联接（用于创建排列或行的组合）将其级联。 这样，我计算出成本是在联接O（row ^ joins）之后调用它的O（n）。 首先是这样的：

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

我想做类似下面的事情。 我可以对select进行临时表操作，然后将其联接起来，但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 ...

我希望做类似的事情：

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

要获得这样的东西：

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

任何帮助将不胜感激。

Answer 1

无需创建临时表，只需将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 ...

或具有公共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 ...

Answer 2

听起来像是你需要使用的窗函数看到这个例子。

例：

SELECT id, sum(cost) OVER (ORDER BY cost) FROM t1;