mysql和并结果集

SELECT COUNT(*) AS total_orders FROM orders
UNION
SELECT COUNT(*) AS total_orders FROM archived_orders;

-- 错误示例
SELECT order_id, order_date FROM orders
UNION
SELECT order_id FROM archived_orders; -- 列数不匹配

-- 正确示例
SELECT order_id, order_date FROM orders
UNION
SELECT order_id, NULL AS order_date FROM archived_orders; -- 添加NULL填充

-- 错误示例
SELECT order_id, CAST(order_date AS VARCHAR(20)) FROM orders
UNION
SELECT order_id, order_date FROM archived_orders; -- 数据类型不兼容

-- 正确示例
SELECT order_id, order_date FROM orders
UNION
SELECT order_id, CAST(order_date AS DATETIME) FROM archived_orders; -- 数据类型一致

-- 示例：分页查询
SELECT * FROM (
    SELECT order_id, order_date FROM orders
    UNION ALL
    SELECT order_id, order_date FROM archived_orders
) AS combined_orders
LIMIT 10 OFFSET 0;