结论是：依次执行。
示例
准备数据
CREATE TABLE IF NOT EXISTS `tb_test`(
`id` INT AUTO_INCREMENT,
`num` INT NOT NULL,
PRIMARY KEY ( `id` )
)ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
insert into tb_test(num) values(1);
更新示例
MySQL root@localhost:mysql> select * from tb_test;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
+----+-----+
MySQL root@localhost:mysql> update tb_test set num = 1, num =2 where id = 1;
Query OK, 1 row affected
Time: 0.005s
MySQL root@localhost:mysql> select * from tb_test;
+----+-----+
| id | num |
+----+-----+
| 1 | 2 |
+----+-----+
1 row in set
Time: 0.005s
MySQL root@localhost:mysql> update tb_test set num = 2, num =1 where id = 1;
Query OK, 1 row affected
Time: 0.004s
MySQL root@localhost:mysql> select * from tb_test;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
+----+-----+
MySQL root@localhost:mysql> update tb_test set num = 1, num = num + 1 where id = 1;
Query OK, 1 row affected
Time: 0.003s
MySQL root@localhost:mysql> select * from tb_test;
+----+-----+
| id | num |
+----+-----+
| 1 | 2 |
+----+-----+
1 row in set
Time: 0.005s
MySQL root@localhost:mysql> update tb_test set num = num + 1, num =20 where id = 1;
Query OK, 1 row affected
Time: 0.004s
MySQL root@localhost:mysql> select * from tb_test;
+----+-----+
| id | num |
+----+-----+
| 1 | 20 |
+----+-----+
1 row in set
Time: 0.005s