在 MySQL 數據庫中，查詢拆分是一種優化技術，它將一個復雜的查詢拆分成多個簡單的查詢，然后將結果組合在一起，以提高查詢性能。以下是一些關于 MySQL 查詢拆分的示例和說明。

/* 示例 1：將一個復雜查詢拆分成兩個簡單查詢 */
-- 第一個查詢
SELECT customer_id, SUM(amount) AS total_spent
FROM orders
GROUP BY customer_id
HAVING total_spent >1000;
-- 第二個查詢
SELECT customer_id, AVG(amount) AS average_spent
FROM orders
GROUP BY customer_id
HAVING average_spent< 50;
/* 示例 2：將查詢拆分成并行執行的多個查詢 */
-- 查詢 1
SELECT *
FROM orders
WHERE order_date BETWEEN '2020-01-01' AND '2020-03-31';
-- 查詢 2
SELECT *
FROM orders
WHERE order_date BETWEEN '2020-04-01' AND '2020-06-30';
-- 查詢 3
SELECT *
FROM orders
WHERE order_date BETWEEN '2020-07-01' AND '2020-09-30';
-- 并行執行上述三個查詢
/* 示例 3：將一個查詢拆分成兩個查詢（使用 UNION ALL 組合結果） */
-- 第一個查詢
SELECT customer_id, SUM(amount) AS total_spent
FROM orders
GROUP BY customer_id
HAVING total_spent >1000;
-- 第二個查詢
SELECT customer_id, COUNT(*) AS num_orders
FROM orders
GROUP BY customer_id
HAVING num_orders >3;
-- 使用 UNION ALL 組合兩個查詢的結果
SELECT customer_id, total_spent, num_orders
FROM (
SELECT customer_id, SUM(amount) AS total_spent, NULL AS num_orders
FROM orders
GROUP BY customer_id
HAVING total_spent >1000
UNION ALL
SELECT customer_id, NULL AS total_spent, COUNT(*) AS num_orders
FROM orders
GROUP BY customer_id
HAVING num_orders >3
) AS intermediate_table;