MySQL基础操作指南：为数据管理打下坚实基础

在MySQL中，CONNECT BY START WITH 是 Oracle 数据库中用于实现层次查询（Hierarchical Query）的语法，而 MySQL 并不直接支持这种语法。不过，我们可以使用递归公用表表达式（Common Table Expressions, CTEs）来实现类似的功能。

从 MySQL 8.0 开始，引入了递归 CTE，这使得我们可以模拟层次查询。下面是一个示例，展示了如何使用递归 CTE 来实现 Oracle 中 CONNECT BY START WITH 的功能。

假设我们有一个员工表 employees，结构如下：

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    manager_id INT
);
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    manager_id INT
);
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    manager_id INT
);

并且表中有以下数据：

INSERT INTO employees (id, name, manager_id) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 2),
(6, 'Frank', 3);
INSERT INTO employees (id, name, manager_id) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 2),
(6, 'Frank', 3);
INSERT INTO employees (id, name, manager_id) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 2),
(6, 'Frank', 3);

在这个表中，id 是员工的唯一标识，name 是员工的名字，manager_id 是员工的经理的 id。我们希望查询出所有员工及其上级经理的层次结构。

可以使用递归 CTE 来实现这个查询：

WITH RECURSIVE employee_hierarchy AS (
    -- 基础查询，相当于 START WITH
    SELECT 
        id, 
        name, 
        manager_id,
        CAST(name AS CHAR(255)) AS path,
        1 AS level
    FROM 
        employees
    WHERE 
        manager_id IS NULL
    UNION ALL
    -- 递归部分，相当于 CONNECT BY
    SELECT 
        e.id, 
        e.name, 
        e.manager_id,
        CONCAT(eh.path, ' -> ', e.name) AS path,
        eh.level + 1 AS level
    FROM 
        employees e
    INNER JOIN 
        employee_hierarchy eh ON e.manager_id = eh.id
)
SELECT 
    id, 
    name, 
    manager_id,
    path,
    level
FROM 
    employee_hierarchy
ORDER BY 
    path;
WITH RECURSIVE employee_hierarchy AS (
    -- 基础查询，相当于 START WITH
    SELECT 
        id, 
        name, 
        manager_id,
        CAST(name AS CHAR(255)) AS path,
        1 AS level
    FROM 
        employees
    WHERE 
        manager_id IS NULL

    UNION ALL

    -- 递归部分，相当于 CONNECT BY
    SELECT 
        e.id, 
        e.name, 
        e.manager_id,
        CONCAT(eh.path, ' -> ', e.name) AS path,
        eh.level + 1 AS level
    FROM 
        employees e
    INNER JOIN 
        employee_hierarchy eh ON e.manager_id = eh.id
)
SELECT 
    id, 
    name, 
    manager_id,
    path,
    level
FROM 
    employee_hierarchy
ORDER BY 
    path;
WITH RECURSIVE employee_hierarchy AS (
    -- 基础查询，相当于 START WITH
    SELECT 
        id, 
        name, 
        manager_id,
        CAST(name AS CHAR(255)) AS path,
        1 AS level
    FROM 
        employees
    WHERE 
        manager_id IS NULL

    UNION ALL

    -- 递归部分，相当于 CONNECT BY
    SELECT 
        e.id, 
        e.name, 
        e.manager_id,
        CONCAT(eh.path, ' -> ', e.name) AS path,
        eh.level + 1 AS level
    FROM 
        employees e
    INNER JOIN 
        employee_hierarchy eh ON e.manager_id = eh.id
)
SELECT 
    id, 
    name, 
    manager_id,
    path,
    level
FROM 
    employee_hierarchy
ORDER BY 
    path;

在这个查询中：

1. WITH RECURSIVE employee_hierarchy AS (...) 定义了递归 CTE。
2. 第一个 SELECT 语句（基础查询）选择顶层员工（即没有经理的员工），这相当于 Oracle 中的 START WITH 部分。
3. UNION ALL 将基础查询的结果与递归部分的结果合并。
4. 递归部分选择每个员工的上级经理，并将路径和层级信息添加到结果集中。
5. 最终的 SELECT 语句从递归 CTE 中选择所需的数据，并按路径排序以显示层次结构。

运行这个查询，你会得到类似如下的结果：

+----+---------+------------+-----------------------+-------+
| id | name    | manager_id | path                  | level |
+----+---------+------------+-----------------------+-------+
|  1 | Alice   |       NULL | Alice                 |     1 |
|  2 | Bob     |          1 | Alice -> Bob          |     2 |
|  3 | Charlie |          1 | Alice -> Charlie      |     2 |
|  4 | David   |          2 | Alice -> Bob -> David |     3 |
|  5 | Eve     |          2 | Alice -> Bob -> Eve   |     3 |
|  6 | Frank   |          3 | Alice -> Charlie -> Frank |     3 |
+----+---------+------------+-----------------------+-------+
+----+---------+------------+-----------------------+-------+
| id | name    | manager_id | path                  | level |
+----+---------+------------+-----------------------+-------+
|  1 | Alice   |       NULL | Alice                 |     1 |
|  2 | Bob     |          1 | Alice -> Bob          |     2 |
|  3 | Charlie |          1 | Alice -> Charlie      |     2 |
|  4 | David   |          2 | Alice -> Bob -> David |     3 |
|  5 | Eve     |          2 | Alice -> Bob -> Eve   |     3 |
|  6 | Frank   |          3 | Alice -> Charlie -> Frank |     3 |
+----+---------+------------+-----------------------+-------+
+----+---------+------------+-----------------------+-------+
| id | name    | manager_id | path                  | level |
+----+---------+------------+-----------------------+-------+
|  1 | Alice   |       NULL | Alice                 |     1 |
|  2 | Bob     |          1 | Alice -> Bob          |     2 |
|  3 | Charlie |          1 | Alice -> Charlie      |     2 |
|  4 | David   |          2 | Alice -> Bob -> David |     3 |
|  5 | Eve     |          2 | Alice -> Bob -> Eve   |     3 |
|  6 | Frank   |          3 | Alice -> Charlie -> Frank |     3 |
+----+---------+------------+-----------------------+-------+

这样，我们就用递归 CTE 在 MySQL 中实现了类似于 Oracle CONNECT BY START WITH 的层次查询功能。