MySQL UPDATE 更新
用 UPDATE 修改已有数据，牢记先加 WHERE。
Key PointsUPDATE 一定要带 WHERE，否则会把整张表改掉——这是生产事故第一名。
基本语法
SQL
UPDATE users
SET age = 29, email = 'alice2@example.com'
WHERE username = 'alice';Output
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0不加 WHERE 的后果
SQL
UPDATE users SET age = 0; -- 全表年龄清零！Output
Query OK, 4 rows affected (0.01 sec)
Rows matched: 4 Changed: 4 Warnings: 0Before UPDATE/DELETE, run
SELECT COUNT(*) with the same WHERE to confirm the row count, then switch to the write. MySQL 8.0 also supports UPDATE ... ORDER BY ... LIMIT n.基本语法
SQL
UPDATE users
SET age = 29, email = 'alice2@example.com'
WHERE username = 'alice';Output
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0不加 WHERE 的后果
SQL
UPDATE users SET age = 0; -- 全表年龄清零！Output
Query OK, 4 rows affected (0.01 sec)
Rows matched: 4 Changed: 4 Warnings: 0执行 UPDATE / DELETE 前，先用相同的 WHERE 跑一遍
SELECT COUNT(*) 确认影响行数符合预期，再改成写操作。MySQL 8.0 还支持 UPDATE ... ORDER BY ... LIMIT n 只改前 n 行。