name	sql-expert
description	SQL 专家技能，帮助编写、优化和调试 SQL 查询
category	database
tags	["sql","database","query","optimization","mysql","postgresql"]

SQL 专家技能

概述

这个技能帮助你处理各种 SQL 相关任务：

编写复杂 SQL 查询
优化查询性能
数据库设计建议
SQL 语法调试
跨数据库方言转换

支持的数据库

MySQL / MariaDB
PostgreSQL
SQLite
SQL Server
Oracle

使用方法

1. 查询编写

根据需求编写 SQL 查询：

简单查询

-- 基本查询
SELECT column1, column2
FROM table_name
WHERE condition
ORDER BY column1 DESC
LIMIT 10;

多表关联

-- JOIN 查询
SELECT 
    u.name,
    o.order_id,
    o.total_amount
FROM users u
INNER JOIN orders o ON u.id = o.user_id
WHERE o.created_at >= '2024-01-01'
ORDER BY o.total_amount DESC;

聚合分析

-- 分组统计
SELECT 
    DATE(created_at) as date,
    COUNT(*) as order_count,
    SUM(amount) as total_amount,
    AVG(amount) as avg_amount
FROM orders
WHERE status = 'completed'
GROUP BY DATE(created_at)
HAVING COUNT(*) > 10
ORDER BY date DESC;

窗口函数

-- 排名和累计
SELECT 
    product_name,
    sales,
    RANK() OVER (ORDER BY sales DESC) as rank,
    SUM(sales) OVER (ORDER BY date ROWS UNBOUNDED PRECEDING) as cumulative_sales
FROM product_sales;

2. 查询优化

优化慢查询的常用技巧：

添加索引

-- 为常用查询条件创建索引
CREATE INDEX idx_orders_user_date ON orders(user_id, created_at);

-- 复合索引
CREATE INDEX idx_products_category_price ON products(category_id, price);

查询重写

-- 避免 SELECT *
-- 不好
SELECT * FROM users WHERE id = 1;

-- 好
SELECT id, name, email FROM users WHERE id = 1;

-- 避免在 WHERE 中使用函数
-- 不好
SELECT * FROM orders WHERE YEAR(created_at) = 2024;

-- 好
SELECT * FROM orders WHERE created_at >= '2024-01-01' AND created_at < '2025-01-01';

使用 EXPLAIN 分析

EXPLAIN ANALYZE
SELECT u.name, COUNT(o.id) as order_count
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
GROUP BY u.id;

3. 常用模式

分页查询

-- 偏移分页（适合小数据量）
SELECT * FROM products
ORDER BY id
LIMIT 20 OFFSET 40;

-- 游标分页（适合大数据量）
SELECT * FROM products
WHERE id > :last_id
ORDER BY id
LIMIT 20;

递归查询（树形结构）

-- PostgreSQL / MySQL 8+
WITH RECURSIVE category_tree AS (
    SELECT id, name, parent_id, 0 as level
    FROM categories
    WHERE parent_id IS NULL
    
    UNION ALL
    
    SELECT c.id, c.name, c.parent_id, ct.level + 1
    FROM categories c
    INNER JOIN category_tree ct ON c.parent_id = ct.id
)
SELECT * FROM category_tree;

UPSERT（插入或更新）

-- MySQL
INSERT INTO users (id, name, email)
VALUES (1, 'John', 'john@example.com')
ON DUPLICATE KEY UPDATE
    name = VALUES(name),
    email = VALUES(email);

-- PostgreSQL
INSERT INTO users (id, name, email)
VALUES (1, 'John', 'john@example.com')
ON CONFLICT (id) DO UPDATE SET
    name = EXCLUDED.name,
    email = EXCLUDED.email;

输出格式

SQL 建议应包含：

## SQL 查询建议

### 查询语句
```sql
[优化后的 SQL]

说明

查询目的: [描述]
预期性能: [估计]

索引建议

[建议创建的索引]

注意事项

[注意点1]
[注意点2]


## 性能检查清单

- [ ] 是否使用了 SELECT *？
- [ ] WHERE 条件是否可以使用索引？
- [ ] JOIN 条件是否有索引？
- [ ] 是否有不必要的子查询？
- [ ] 是否可以使用 EXISTS 替代 IN？
- [ ] 分页是否使用了高效方式？
- [ ] 是否有 N+1 查询问题？