SQL 中的递归 [英] RECURSIVE in SQL

问题描述

我正在学习 SQL 并且很难理解以下递归 SQL 语句.

I'm learning SQL and had a hard time understanding the following recursive SQL statement.

WITH RECURSIVE t(n) AS (
    SELECT 1
    UNION ALL
    SELECT n+1 FROM t WHERE n < 100
)
SELECT sum(n) FROM t;

SELECT sum(n) FROM t; 中的 n 和 t 是什么?据我所知，n 是一个数，t 是一个集合.我说得对吗?

What is n and t from SELECT sum(n) FROM t;? As far as I could understand, n is a number of t is a set. Am I right?

还有这个语句中的递归是怎么触发的?

Also how is recursion triggered in this statement?

推荐答案

您使用的语法看起来像 Postgres.SQL 中的递归"并不是真正的递归，而是迭代.您的陈述是:

The syntax that you are using looks like Postgres. "Recursion" in SQL is not really recursion, it is iteration. Your statement is:

WITH RECURSIVE t(n) AS (
    SELECT 1
    UNION ALL
    SELECT n+1 FROM t WHERE n < 100
)
SELECT sum(n) FROM t;

t 的语句被评估为:

1. 评估非自引用部分(select 1).
2. 然后评估自我引用部分.(最初这给出了 2.)
3. 然后再次评估自引用部分.(3).
4. 依此类推，条件仍然有效(n <100).

完成后，t 子查询就完成了，可以评估最终语句.

When this is done the t subquery is finished, and the final statement can be evaluated.

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