11、（5-8） choose the best answer:
Examine the structure of the BOOKS_TRANSACTIONS table.
You want to update this table such that BOOK_ID is set to 'INVALID' for all rows where no MEMBER_ID
has been entered.
Examine this partial SQL statement:
SQL> UPDATE books_transactions
SET book_id = 'INVALID'
WHERE...............
Which condition must be used in the WHERE clause to perform the required update?
A) MEMBER_ID = "";
B) MEMBER_ID IS NULL;
C) MEMBER_ID = NULL;
D) MEMBER_ID = '';
Answer：B
（解析：该题的题眼在“where no MEMBER_ID has been entered”）