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:

(題意：題目給了一個發貨表Shipments，其中有PO_DATE和SHIPMENT_DATE 欄位，如果SHIPMENT_DATE比PO_DATE遲一個月，則每多一天罰款20，對此，請評價下面給出的兩個sql語句)

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函式的語法是，decode(條件,值1,返回值1,值2,返回值2,...值n,返回值n,預設值)

對比，上面decode使用錯誤。Case...When的語法是正確的，並且語句寫的也是正確的，可以得到正確的結果。

第二個decode語法可以這樣改，使用sign函式判斷大小。 

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;