43、(9-2)choose three
Which three tasks can be performed by DDL statements?
A) preventing data retrieval from a table outside of office hours
B) modifying a table to prevent data that violate certain conditions from being entered in a column
C) preventing any data modification to a table
D) creating multiple savepoints to enable partial rollback of a transaction
E) providing an alternative name for a table
Anser：BCE
(解析：B 答案可以用添加约束来解决;C 答案可以用 alter 命令把表变成只读模式;E 答案可以通过创建同义词来实现，这道题比较灵活，表达的比较隐晦。)