mysql中union关键字的实现示例

SELECT column1, column2 FROM table1
UNION [ALL | DISTINCT]
SELECT column1, column2 FROM table2;

-- 查询 2023 和 2024 年的订单（假设按年分表）
SELECT order_id, amount, created_at FROM orders_2023
UNION ALL
SELECT order_id, amount, created_at FROM orders_2024
ORDER BY created_at DESC
LIMIT 20;

SELECT 'VIP用户' as user_type, COUNT(*) as cnt FROM users WHERE level >= 8
UNION ALL
SELECT '普通用户', COUNT(*) FROM users WHERE level BETWEEN 1 AND 7
UNION ALL
SELECT '访客', COUNT(*) FROM users WHERE level = 0;

-- 先查缓存表，若无结果再查主表（常见于配置表）
SELECT config_value FROM config_cache WHERE config_key = 'site_name'
UNION ALL
SELECT config_value FROM config WHERE config_key = 'site_name'
LIMIT 1;

(SELECT id, name FROM products WHERE category = 'A' ORDER BY price DESC LIMIT 10)
UNION ALL
(SELECT id, name FROM products WHERE category = 'B' ORDER BY price DESC LIMIT 10)
ORDER BY price DESC LIMIT 10;

SELECT city, sales_amount, NULL as detail FROM sales_summary
UNION ALL
SELECT city, amount, CONCAT(product_name, ':', quantity) FROM sales_detail;

-- ❌ 错误：列数不匹配
SELECT id, name FROM t1
UNION
SELECT id FROM t2;  -- ERROR 1222

-- ❌ 错误：ORDER BY 位置错误
SELECT * FROM t1 ORDER BY id
UNION
SELECT * FROM t2;  -- ERROR 1221

-- ✅ 正确写法
SELECT * FROM t1
UNION
SELECT * FROM t2
ORDER BY id;  -- 使用第一个 SELECT 的列名排序

from django.db.models import Q

# QuerySet.union() 默认使用 UNION ALL
qs1 = User.objects.filter(status='active').values('name', 'email')
qs2 = User.objects.filter(is_staff=True).values('name', 'email')

# UNION ALL
combined = qs1.union(qs2, all=True)

# UNION (去重)
combined_distinct = qs1.union(qs2)  # 或 all=False