02. 关系模型与 SQL 基础

Fri, 14 Aug 2026 16:02:00 +0800

1. 关系代数

关系数据库基于关系代数，核心操作：

操作	SQL 对应	说明
σ (选择)	WHERE	按条件过滤行
π (投影)	SELECT	选择列
⋈ (连接)	JOIN	表关联
∪ (并)	UNION	合并结果集
− (差)	EXCEPT	差集
ρ (重命名)	AS	别名

2. SQL 核心语法

-- 条件聚合
SELECT
 department,
 COUNT(*) AS total_employees,
 COUNT(CASE WHEN salary > 100000 THEN 1 END) AS high_earners,
 AVG(salary) FILTER (WHERE status = 'active') AS avg_salary
FROM employees
GROUP BY department
HAVING COUNT(*) > 5;

-- 窗口函数
SELECT
 name,
 department,
 salary,
 RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS dept_rank,
 salary - AVG(salary) OVER (PARTITION BY department) AS diff_from_avg,
 SUM(salary) OVER (ORDER BY hire_date) AS running_total
FROM employees;

-- CTE 递归查询（组织架构）
WITH RECURSIVE org_tree AS (
 SELECT id, name, manager_id, 0 AS level
 FROM employees WHERE manager_id IS NULL
 UNION ALL
 SELECT e.id, e.name, e.manager_id, ot.level + 1
 FROM employees e
 JOIN org_tree ot ON e.manager_id = ot.id
)
SELECT * FROM org_tree;

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02. 关系模型与 SQL 基础

1. 关系代数

2. SQL 核心语法

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