修改列语句，为什么选B呢？解决方法

修改列语句，为什么选B呢？
The EMPLOYEES table has these columns:

LAST_NAME VARCHAR2(35)
SALARY NUMBER(8,2)
HIRE_DATE DATE
Management wants to add a default value to the SALARY column. You plan to alter the table by using
this SQL statement:
ALTER TABLE EMPLOYEES
MODIFY (SALARY DEFAULT 5000);
Which is true about your ALTER statement?

A. Column definitions cannot be altered to add DEFAULT values.
B. A change to the DEFAULT value affects only subsequent insertions to the table.
C. Column definitions cannot be altered to add DEFAULT values for columns with a NUMBER data type.
D. All the rows that have a NULL value for the SALARY column will be updated with the value 5000.
Answer: B

------解决方案--------------------------------------------------------

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引用楼主 manlibie 的帖子:
The EMPLOYEES table has these columns:

LAST_NAME VARCHAR2(35)
SALARY NUMBER(8,2)
HIRE_DATE DATE
Management wants to add a default value to the SALARY column. You plan to alter the table by using
this SQL statement:
ALTER TABLE EMPLOYEES
MODIFY (SALARY DEFAULT 5000);
Which is true about your ALTER statement?

A. Column definitions cannot be altered …

------解决方案--------------------------------------------------------
B. A change to the DEFAULT value affects only subsequent insertions to the table. 
默认值仅对新插入的记录有效
C. Column definitions cannot be altered to add DEFAULT values for columns with a NUMBER data type. 
栏位定义默认值时,默认值不能加到已存在值的栏位里

译不太好;
两个语句在理解上觉得意思一样,但觉得B更有说服力