for update 和 lock in share mode

共同点：两者都必须在事务中使用
不同点：for update 对记录加写锁，此时记录不能被其他线程加读锁或者写锁。
lock in share mode对记录加读锁，此时记录能被其他线程加读锁，不能加写锁。但是可以在当前事务内，再加写锁。

mysql> begin;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from test where id=7 lock in share mode;
+----+----------+-------------+----------+------+----------+
| id | testEnum | testdecimal | sizetest | date | dateCopy |
+----+----------+-------------+----------+------+----------+
|  7 |        2 |        NULL |     NULL | NULL | NULL     |
+----+----------+-------------+----------+------+----------+
1 row in set (0.00 sec)

mysql> select * from test where id=7 for update;
+----+----------+-------------+----------+------+----------+
| id | testEnum | testdecimal | sizetest | date | dateCopy |
+----+----------+-------------+----------+------+----------+
|  7 |        2 |        NULL |     NULL | NULL | NULL     |
+----+----------+-------------+----------+------+----------+
1 row in set (0.00 sec)