问题描述
是否可以通过arel
在单个查询中组合多个CTE?我正在寻找获得这样结果的方法:
Is it possible to combine multiple CTEs in single query with arel
? I am looking for way to get result like this:
WITH 'cte1' AS (
...
),
WITH RECURSIVE 'cte2' AS (
...
),
WITH 'cte3' AS (
...
)
SELECT ... FROM 'cte3' WHERE ...
如您所见,我有一个递归CTE和两个非递归CTE.
As you can see, I have one recursive CTE and two non recursive.
推荐答案
在顶部使用关键字WITH
一次.如果您的任何通用表表达式(CTE)是递归(rCTE),则即使并非所有CTE都是递归的,您也必须在顶部一次处添加关键字RECURSIVE
:
Use the key word WITH
once at the top. If any of your Common Table Expressions (CTE) are recursive (rCTE) you have to add the keyword RECURSIVE
at the top once also, even if not all CTEs are recursive:
WITH RECURSIVE
cte1 AS (...) -- can still be non-recursive
, cte2 AS (SELECT ...
UNION ALL
SELECT ...) -- recursive term
, cte3 AS (...)
SELECT ... FROM cte3 WHERE ...
强调粗体.而且,更有见识:
Bold emphasis mine. And, even more insightful:
再次强调我的粗体.这意味着使用RECURSIVE
关键字时WITH
子句的顺序是无意义.
Bold emphasis mine again. Meaning that the order of WITH
clauses is meaningless when the RECURSIVE
key word has been used.
BTW,因为示例中的cte1
和cte2
不在外部SELECT
中引用,并且它们本身是普通的SELECT
命令(无附带影响),所以它们从不执行(除非在cte3
中引用) ).
BTW, since cte1
and cte2
in the example are not referenced in the outer SELECT
and are plain SELECT
commands themselves (no collateral effects), they are never executed (unless referenced in cte3
).
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