oracle 递归查询
时间: 2023-06-02 11:02:28 浏览: 116
Oracle 支持递归查询,也称为“递归子查询”或“递归查询”。它允许您查询具有自引用关系的表,例如树形结构或组织结构。以下是一个简单的例子:
假设有一个员工表,其中每个员工都有一个上级,除了最高级别的员工外,他们的上级为空。该表的结构如下:
```
CREATE TABLE employees (
employee_id NUMBER,
employee_name VARCHAR2(100),
manager_id NUMBER
);
```
要查询某个员工及其所有下属的信息,可以使用递归查询。以下是一个示例查询:
```
WITH subordinates (employee_id, employee_name, manager_id, level) AS (
SELECT employee_id, employee_name, manager_id, 1
FROM employees
WHERE employee_id = 1 -- 假设要查询的员工是 ID 为 1 的员工
UNION ALL
SELECT e.employee_id, e.employee_name, e.manager_id, s.level + 1
FROM employees e, subordinates s
WHERE e.manager_id = s.employee_id
)
SELECT employee_id, employee_name, manager_id, level
FROM subordinates;
```
此查询使用了一个公共表达式(WITH 子句),其中递归地定义了一个子查询(subordinates),它从 employees 表中选择一个初始员工(ID 为 1),然后逐级向下查询其所有下属。联接条件是员工的 manager_id 等于上级员工的 employee_id。在每个级别上,子查询都会将 level 增加 1,以便我们可以在结果中区分各个级别的员工。
请注意,这是一个简单的示例,实际的递归查询可能更复杂。此外,递归查询可能会在处理大型数据集时出现性能问题。在这种情况下,您可能需要使用其他技术,例如嵌套集模型或物化视图。
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