| 用途 | 读取数据 |
|---|---|
| 语法 | SELECT 列表 [FROM 表] [WHERE 条件] [GROUP BY …] [ORDER BY …] [LIMIT n] |
| 要点 | 列投影越窄越好 |
常用示例
常用示例
SELECT id, sku, qty, price FROM orders;
SELECT id AS order_id, qty * price AS amount FROM orders;
SELECT *, 1 AS flag FROM orders LIMIT 1;
SELECT CONCAT('ORD-', id) AS order_no FROM orders;执行顺序（书写顺序不同）
执行顺序（书写顺序不同）
FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMITSELECT * 在宽表或含大字段时会把不需要的数据全部拉出来，覆盖索引也无法生效优化。