6. Examine the structure of the SHIPMENTS table:
name Null Type
PO_ID NOT NULL NUMBER(3)
PO_DATE NOT NULL DATE
SHIPMENT_DATE NOT NULL DATE
SHIPMENT_MODE VARCHAR2(30)
SHIPMENT_COST NUMBER(8,2)
You want to generate a report that displays the PO_ID and the penalty amount to be paid if the
SHIPMENT_DATE is later than one month from the PO_DATE. The penalty is $20 per day.
Evaluate the following two queries:
(题意：题目给了一个发货表Shipments，当中有PO_DATE和SHIPMENT_DATE 字段，假设SHIPMENT_DATE比PO_DATE迟一个月，则每多一天罚款20，对此，请评价以下给出的两个sql语句)
SQL> SELECT po_id, CASE
WHEN MONTHS_BETWEEN (shipment_date,po_date)>1 THEN
TO_CHAR((shipment_date - po_date) * 20) ELSE 'No Penalty' END PENALTY
FROM shipments;
SQL>SELECT po_id, DECODE
(MONTHS_BETWEEN (po_date,shipment_date)>1,
TO_CHAR((shipment_date - po_date) * 20), 'No Penalty') PENALTY
FROM shipments;
Which statement is true regarding the above commands?
A. Both execute successfully and give correct results.
B. Only the first query executes successfully but gives a wrong result.
C. Only the first query executes successfully and gives the correct result.
D. Only the second query executes successfully but gives a wrong result.
E. Only the second query executes successfully and gives the correct result.
Answer: C
decode函数的语法是，decode(条件,值1,返回值1,值2,返回值2,...值n,返回值n,缺省值)
对照。上面decode使用错误。Case...When的语法是正确的，而且语句写的也是正确的，能够得到正确的结果。
第二个decode语法能够这样改，使用sign函数推断大小。
1 SELECT po_id, DECODE
2 (SIGN(MONTHS_BETWEEN(po_date,shipment_date)),1
3 TO_CHAR((shipment_date - po_date) * 20), 'No Penalty') PENALTY
4 FROM shipments;