163. View the Exhibit and examine the structure of ORD and ORD_ITEMS tables. In the ORD table, the PRIMARY KEY is ORD_NO and in the ORD_ITEMS tables the composite PRIMARY KEY is (ORD_NO, ITEM_NO). Which two CREATE INDEX statements are valid? (Choose two.)
A. CREATE INDEX ord_idx ON ord(ord_no);
B. CREATE INDEX ord_idx ON ord_items(ord_no);
C. CREATE INDEX ord_idx ON ord_items(item_no);
D. CREATE INDEX ord_idx ON ord,ord_items(ord_no, ord_date,qty);
Answer: BC
个人觉得 d的 语法是错了，可是 A 错在哪里？
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ord_no是ord的主键，无须再建索引吧
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刚试了下，会报索引已存在的