WITH RECURSIVE hierarchy

`WITH RECURSIVE` 是 SQL 中的一种特性，特别用于编写递归查询（Recursive Query）。当数据存在层级关系，比如树形结构时，这种语法可以帮助你分层次地遍历数据并生成完整的关系图。在 `WITH RECURSIVE` 语句中，你会定义一个或多个基础（基础阶段）和扩展（递归阶段）部分。

例如，在 `hierarchy` 这样的上下文中，基础部分通常包括初始的起点（通常是无父节点的记录），而扩展部分则基于已有的结果进行递归查询，通过 `JOIN` 或者 `UNION ALL` 将当前层的记录与上一层关联起来。这个过程会一直持续，直到满足某个停止条件，如没有更多的父节点或者达到预设的最大层数。

举个例子，如果我们要创建一个查询来找出公司的所有部门及其所有下属，可能的 `WITH RECURSIVE` 语句如下：

WITH RECURSIVE department_hierarchy AS (
  -- 基础阶段：选择根部（无父节点）
  SELECT d.*
  FROM departments d
  WHERE d.parent_id IS NULL

  -- 递归阶段：将每个部门与其子部门合并
  UNION ALL
  SELECT d.*
  FROM departments d
  JOIN department_hierarchy dh ON d.parent_id = dh.id
)
SELECT * FROM department_hierarchy;

这个查询将会返回一个完整的部门层级结构。

相关问题

with recursive

With recursive is a clause used in SQL queries to perform recursive operations on hierarchical data structures. It allows you to repeatedly query a table and its related tables, building up a result set by combining the results of each iteration.

By using the WITH RECURSIVE clause, you can define a recursive query that consists of two parts: the base case and the recursive case. The base case represents the starting point of the recursion, while the recursive case defines how to transform the result set from the previous iteration into the result set for the current iteration.

Here's an example to illustrate how it works:

WITH RECURSIVE RecursiveCTE (id, name, parent_id, level) AS (
  SELECT id, name, parent_id, 0 
  FROM employees 
  WHERE name = 'John'
  
  UNION ALL
  
  SELECT e.id, e.name, e.parent_id, rc.level + 1
  FROM employees e
  INNER JOIN RecursiveCTE rc ON e.parent_id = rc.id
)
SELECT id, name, level
FROM RecursiveCTE;

In this example, we have an "employees" table with columns like "id", "name", and "parent_id". We want to find all the employees that are in the hierarchical chain above "John", including "John" himself. The recursive query starts with the base case where we select "John" and set the initial level to 0. Then, in the recursive case, we join the "employees" table with the RecursiveCTE (the previous iteration of the query) to find employees who have their parent_id equal to the id of the previous iteration. The recursion continues until no more matches are found.

The final result of the query is a result set that includes all the employees in the hierarchical chain above "John", along with their respective levels in the hierarchy.

Note that the specific syntax and capabilities of the WITH RECURSIVE clause may vary depending on the database management system you are using.

求一个WITH RECURSIVE的例子

WITH RECURSIVE是一种递归查询语法，可以用于处理具有分层结构的数据。以下是一个简单的例子：

假设我们有一个员工表（employees），其中包含每个员工的ID、姓名和上级的ID。我们想要查询每个员工的名字以及他们的上级的名字，以便可以形成一个员工层次结构。我们可以使用WITH RECURSIVE来实现这个查询：

WITH RECURSIVE emp_hierarchy AS (
  SELECT id, name, manager_id
  FROM employees
  WHERE id = 1   -- 假设我们从ID为1的员工开始查询
  UNION ALL
  SELECT e.id, e.name, e.manager_id
  FROM employees e
  JOIN emp_hierarchy eh ON e.id = eh.manager_id
)
SELECT name, manager_id
FROM emp_hierarchy;

在这个例子中，我们定义了一个名为“emp_hierarchy”的递归查询，它包含三个列：员工ID、员工姓名和上级ID。我们从ID为1的员工开始查询，并通过UNION ALL操作递归地加入每个上级的信息，直到没有更多上级为止。最后，我们从递归查询中选择员工姓名和上级ID。

注意，递归查询必须包含一个初始查询，它需要返回一些结果，以便递归操作可以开始。在这个例子中，我们从ID为1的员工开始查询。