ORACLE LEVEL函数练习

Oracle LEVEL 专项练习（必练 10 题）

LEVEL 是 Oracle 树形查询专属伪列 ，表示当前递归层级，根节点 = 1，每递归一层 + 1。

先建测试表（所有练习通用）

sql

CREATE TABLE emp (
    emp_id NUMBER PRIMARY KEY,
    name VARCHAR2(20),
    mgr_id NUMBER
);

INSERT INTO emp VALUES(1, '董事长', NULL);
INSERT INTO emp VALUES(2, '总经理', 1);
INSERT INTO emp VALUES(3, '销售总监', 2);
INSERT INTO emp VALUES(4, '华北经理', 3);
INSERT INTO emp VALUES(5, '华南经理', 3);
INSERT INTO emp VALUES(6, '北京销售', 4);
INSERT INTO emp VALUES(7, '深圳销售', 5);

COMMIT;

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练习 1：最基础 ------ 显示每个人的层级

sql

SELECT
    emp_id,
    name,
    mgr_id,
    LEVEL 层级
FROM emp
START WITH mgr_id IS NULL
CONNECT BY PRIOR emp_id = mgr_id;

目标：理解 LEVEL 从 1 开始递增。

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练习 2：只查第 2 层（总经理）

sql

SELECT *
FROM (
    SELECT name, LEVEL 层级
    FROM emp
    START WITH mgr_id IS NULL
    CONNECT BY PRIOR emp_id = mgr_id
)
WHERE 层级 = 2;

---

练习 3：只查叶子节点（最底层员工）

sql

SELECT name, LEVEL 层级
FROM emp
WHERE CONNECT_BY_ISLEAF = 1  -- 1=叶子节点
START WITH mgr_id IS NULL
CONNECT BY PRIOR emp_id = mgr_id;

---

练习 4：只查3 级及以下

sql

SELECT name, LEVEL 层级
FROM emp
WHERE LEVEL <= 3
START WITH mgr_id IS NULL
CONNECT BY PRIOR emp_id = mgr_id;

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练习 5：按层级缩进显示（树形排版）

sql

SELECT
    LPAD(' ', 2*(LEVEL-1)) || name AS 树形结构,
    LEVEL 层级
FROM emp
START WITH mgr_id IS NULL
CONNECT BY PRIOR emp_id = mgr_id;

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练习 6：向上递归查上级，LEVEL 反向计数

sql

-- 查"北京销售"所有上级，LEVEL 从自己开始=1
SELECT name, LEVEL
FROM emp
START WITH name = '北京销售'
CONNECT BY PRIOR mgr_id = emp_id;

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练习 7：用 LEVEL 生成连续数字（面试高频）

不用表，直接生成 1~10：

sql

SELECT LEVEL AS num
FROM dual
CONNECT BY LEVEL <= 10;

生成 5~15：

sql

SELECT LEVEL+4 AS num
FROM dual
CONNECT BY LEVEL <= 11;

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练习 8：生成连续日期（LEVEL 超级常用）

生成未来 7 天：

sql

SELECT
    SYSDATE + LEVEL - 1 AS dt,
    LEVEL
FROM dual
CONNECT BY LEVEL <= 7;

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练习 9：生成多行重复数据（测试数据专用）

sql

SELECT '测试数据' || LEVEL AS txt, LEVEL
FROM dual
CONNECT BY LEVEL <= 5;

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练习 10：限定递归深度（最多递归 2 层）

sql

SELECT name, LEVEL
FROM emp
START WITH mgr_id IS NULL
CONNECT BY PRIOR emp_id = mgr_id
       AND LEVEL <= 2; -- 只到总经理

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LEVEL 核心口诀

1. 根节点 LEVEL=1
2. 向下递归：PRIOR 子ID = 父ID
3. 向上递归：PRIOR 父ID = 子ID
4. 生成序列 / 日期：CONNECT BY LEVEL <= N
5. 过滤层级：WHERE LEVEL = x 或 LEVEL <= x