| 用途 | 聚合统计 |
|---|---|
| 函数 | COUNT / SUM / AVG / MAX / MIN / GROUP_CONCAT |
| 语义 | 除 COUNT(*) 外都忽略 NULL |
常用示例
常用示例
SELECT COUNT(*) AS rows_cnt, COUNT(email) AS email_cnt FROM users;
SELECT SUM(amount) AS total, AVG(amount) AS avg_amt FROM orders WHERE status='paid';
SELECT MAX(created_at) AS last_order FROM orders;GROUP_CONCAT 合并行
GROUP_CONCAT 合并行
SELECT user_id, GROUP_CONCAT(sku ORDER BY id SEPARATOR ',') AS skus
FROM order_items GROUP BY user_id;COUNT(列) 只统计非 NULL 值；GROUP_CONCAT 默认受 group_concat_max_len 限制（默认 1024 字节）会静默截断。