database-schema-designer

范式

第一范式（1NF）

-- BAD: Multiple values in column
CREATE TABLE orders (
  id INT PRIMARY KEY,
  product_ids VARCHAR(255)  -- '101,102,103'
);

-- GOOD: Separate table for items
CREATE TABLE orders (
  id INT PRIMARY KEY,
  customer_id INT
);

CREATE TABLE order_items (
  id INT PRIMARY KEY,
  order_id INT REFERENCES orders(id),
  product_id INT
);

第二范式（2NF）

-- BAD: customer_name depends only on customer_id
CREATE TABLE order_items (
  order_id INT,
  product_id INT,
  customer_name VARCHAR(100),  -- Partial dependency!
  PRIMARY KEY (order_id, product_id)
);

-- GOOD: Customer data in separate table
CREATE TABLE customers (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);

第三范式（3NF）

-- BAD: country depends on postal_code
CREATE TABLE customers (
  id INT PRIMARY KEY,
  postal_code VARCHAR(10),
  country VARCHAR(50)  -- Transitive dependency!
);

-- GOOD: Separate postal_codes table
CREATE TABLE postal_codes (
  code VARCHAR(10) PRIMARY KEY,
  country VARCHAR(50)
);

何时反范式化

-- Denormalized for performance
CREATE TABLE orders (
  id INT PRIMARY KEY,
  customer_id INT,
  total_amount DECIMAL(10,2),  -- Calculated
  item_count INT               -- Calculated
);

字符串类型

-- Good sizing
email VARCHAR(255)
phone VARCHAR(20)
country_code CHAR(2)

数值类型

-- ALWAYS use DECIMAL for money
price DECIMAL(10, 2)  -- $99,999,999.99

-- NEVER use FLOAT for money
price FLOAT  -- Rounding errors!

日期/时间类型

DATE        -- 2025-10-31
TIME        -- 14:30:00
DATETIME    -- 2025-10-31 14:30:00
TIMESTAMP   -- Auto timezone conversion

-- Always store in UTC
created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP

布尔值

-- PostgreSQL
is_active BOOLEAN DEFAULT TRUE

-- MySQL
is_active TINYINT(1) DEFAULT 1

何时创建索引

-- Foreign key index
CREATE INDEX idx_orders_customer ON orders(customer_id);

-- Query pattern index
CREATE INDEX idx_orders_status_date ON orders(status, created_at);

索引类型

复合索引顺序

CREATE INDEX idx_customer_status ON orders(customer_id, status);

-- Uses index (customer_id first)
SELECT * FROM orders WHERE customer_id = 123;
SELECT * FROM orders WHERE customer_id = 123 AND status = 'pending';

-- Does NOT use index (status alone)
SELECT * FROM orders WHERE status = 'pending';

规则： 选择性最高的列在前，或单独查询最多的列在前。

索引陷阱

主键

-- Auto-increment (simple)
id INT AUTO_INCREMENT PRIMARY KEY

-- UUID (distributed systems)
id CHAR(36) PRIMARY KEY DEFAULT (UUID())

-- Composite (junction tables)
PRIMARY KEY (student_id, course_id)

外键

FOREIGN KEY (customer_id) REFERENCES customers(id)
  ON DELETE CASCADE     -- Delete children with parent
  ON DELETE RESTRICT    -- Prevent deletion if referenced
  ON DELETE SET NULL    -- Set to NULL when parent deleted
  ON UPDATE CASCADE     -- Update children when parent changes

其他约束

-- Unique
email VARCHAR(255) UNIQUE NOT NULL

-- Composite unique
UNIQUE (student_id, course_id)

-- Check
price DECIMAL(10,2) CHECK (price >= 0)
discount INT CHECK (discount BETWEEN 0 AND 100)

-- Not null
name VARCHAR(100) NOT NULL

一对多

CREATE TABLE orders (
  id INT PRIMARY KEY,
  customer_id INT NOT NULL REFERENCES customers(id)
);

CREATE TABLE order_items (
  id INT PRIMARY KEY,
  order_id INT NOT NULL REFERENCES orders(id) ON DELETE CASCADE,
  product_id INT NOT NULL,
  quantity INT NOT NULL
);

多对多

-- Junction table
CREATE TABLE enrollments (
  student_id INT REFERENCES students(id) ON DELETE CASCADE,
  course_id INT REFERENCES courses(id) ON DELETE CASCADE,
  enrolled_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (student_id, course_id)
);

自引用

CREATE TABLE employees (
  id INT PRIMARY KEY,
  name VARCHAR(100) NOT NULL,
  manager_id INT REFERENCES employees(id)
);

多态

-- Approach 1: Separate FKs (stronger integrity)
CREATE TABLE comments (
  id INT PRIMARY KEY,
  content TEXT NOT NULL,
  post_id INT REFERENCES posts(id),
  photo_id INT REFERENCES photos(id),
  CHECK (
    (post_id IS NOT NULL AND photo_id IS NULL) OR
    (post_id IS NULL AND photo_id IS NOT NULL)
  )
);

-- Approach 2: Type + ID (flexible, weaker integrity)
CREATE TABLE comments (
  id INT PRIMARY KEY,
  content TEXT NOT NULL,
  commentable_type VARCHAR(50) NOT NULL,
  commentable_id INT NOT NULL
);

嵌入 vs 引用

嵌入式文档

{
  "_id": "order_123",
  "customer": {
    "id": "cust_456",
    "name": "Jane Smith",
    "email": "[email protected]"
  },
  "items": [
    { "product_id": "prod_789", "quantity": 2, "price": 29.99 }
  ],
  "total": 109.97
}

引用文档

{
  "_id": "order_123",
  "customer_id": "cust_456",
  "item_ids": ["item_1", "item_2"],
  "total": 109.97
}

MongoDB 索引

// Single field
db.users.createIndex({ email: 1 }, { unique: true });

// Composite
db.orders.createIndex({ customer_id: 1, created_at: -1 });

// Text search
db.articles.createIndex({ title: "text", content: "text" });

// Geospatial
db.stores.createIndex({ location: "2dsphere" });

迁移最佳实践

添加列（零停机）

-- Step 1: Add nullable column
ALTER TABLE users ADD COLUMN phone VARCHAR(20);

-- Step 2: Deploy code that writes to new column

-- Step 3: Backfill existing rows
UPDATE users SET phone = '' WHERE phone IS NULL;

-- Step 4: Make required (if needed)
ALTER TABLE users MODIFY phone VARCHAR(20) NOT NULL;

重命名列（零停机）

-- Step 1: Add new column
ALTER TABLE users ADD COLUMN email_address VARCHAR(255);

-- Step 2: Copy data
UPDATE users SET email_address = email;

-- Step 3: Deploy code reading from new column
-- Step 4: Deploy code writing to new column

-- Step 5: Drop old column
ALTER TABLE users DROP COLUMN email;

迁移模板

-- Migration: YYYYMMDDHHMMSS_description.sql

-- UP
BEGIN;
ALTER TABLE users ADD COLUMN phone VARCHAR(20);
CREATE INDEX idx_users_phone ON users(phone);
COMMIT;

-- DOWN
BEGIN;
DROP INDEX idx_users_phone ON users;
ALTER TABLE users DROP COLUMN phone;
COMMIT;

查询分析

EXPLAIN SELECT * FROM orders
WHERE customer_id = 123 AND status = 'pending';

N+1 查询问题

# BAD: N+1 queries
orders = db.query("SELECT * FROM orders")
for order in orders:
    customer = db.query(f"SELECT * FROM customers WHERE id = {order.customer_id}")

# GOOD: Single JOIN
results = db.query("""
    SELECT orders.*, customers.name
    FROM orders
    JOIN customers ON orders.customer_id = customers.id
""")

优化技巧