| 用途 | 空值处理 |
|---|---|
| 函数 | IFNULL(a,b) / COALESCE(a,b,c…) / NULLIF(a,b) |
| 判断 | IS NULL / IS NOT NULL |
常用示例
常用示例
SELECT IFNULL(phone, 'N/A') AS phone FROM users;
SELECT COALESCE(cancel_reason, finish_reason, 'unknown') AS reason FROM jobs;
SELECT NULLIF(qty, 0) AS qty_or_null FROM order_items; -- 防止除零
SELECT 1 + NULL AS r; -- 结果为 NULLNULL 参与算术运算结果恒为 NULL；聚合函数（除 COUNT(*) 外）会跳过 NULL，统计口径要写清。