這篇文章對遞迴SQL的抽象和解釋比較到位,摘錄如下,有興趣可參考原文.
```
A general format of a recursive query is as follows:

WITH RECURSIVE t AS (
  non-recursive term      (1)
  UNION ALL
  recursive term          (2)
)
SELECT * FROM t;          (3)

The non-recursive term (1) is performed first. Then the recursive term (2) is iterated while it returns some rows. The recursive term is called so because it can access the output of the previous iteration that is available under the name of t.

In the process, the output of each iteration is placed into a resulting table, which will be available under the same name of t when all the query is complete (3). If we replace UNION ALL with UNION, duplicate rows will be eliminated at each iteration.

Using pseudocode, we may represent this as follows:

res ← EMPTY;
t ← (non-recursive term);
WHILE t IS NOT EMPTY LOOP
    res ← res UNION ALL t;
    aux ← (recursive term);
    t ← aux;
END LOOP;
t ← res;
```

[ On recursive queries ](https://habr.com/en/company/postgrespro/blog/490228/)

PostgreSQL 遞迴查詢

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