ORACLE中的DECODE

Question

我不太明白这个DECODE函数将如何自行解析，特别是在尖头（==＆gt;）参数中：

DECODE
(
    SH.RESPCODE, 0, SH.AMOUNT-DEVICE_FEE,
    102, SH.NEW_AMOUNT-DEVICE_FEE,
    ==>AMOUNT-DEVICE_FEE,
    0
)

提前感谢您对该参数将解决的任何启示。

Answer 1

它说：

If the value of SH.RESPCODE is 0, then return SH.AMOUNT-DEVICE_FEE.
Otherwise, if it's equal to 102, then return SH.NEW_AMOUNT-DEVICE_FEE.
Otherwise, if it's equal to AMOUNT-DEVICE_FEE, then return 0

作为案例陈述，它将是：

Case SH.RESPCODE
  when 0                 then SH.AMOUNT-DEVICE_FEE
  when 102               then rSH.NEW_AMOUNT-DEVICE_FEE
  when AMOUNT-DEVICE_FEE then 0
  else null
end

两者之间在处理NULL方面存在一些细微差别。

Answer 2

在伪代码中，这相当于以下IF语句

IF( sh.respcode = 0 )
THEN
  RETURN sh.amount-device_fee
ELSIF( sh.respcode = 102 )
THEN
  RETURN sh.new_amount-device_fee
ELSIF( sh.respcode = amount-device_fee )
THEN
  RETURN 0;
END IF;

Answer 3

DECODE基本上等同于CASE表达式。

所以这个

DECODE
(
    SH.RESPCODE, 0, SH.AMOUNT-DEVICE_FEE,
    102, SH.NEW_AMOUNT-DEVICE_FEE,
    ==>AMOUNT-DEVICE_FEE,
    0
)

等同于此

CASE SH.RESPCODE
    WHEN 0 THEN SH.AMOUNT - DEVICE_FEE
    WHEN 102 THEN SH.NEW_AMOUNT - DEVICE_FEE
    WHEN AMOUNT - DEVICE_FEE THEN 0
END

Answer 4

DECODE与CASE相当的评论很好。我发现有用的一件事是格式化DECODE调用以使其更具可读性，如：

DECODE(SH.RESPCODE,
         0,                 SH.AMOUNT-DEVICE_FEE,
         102,               SH.NEW_AMOUNT-DEVICE_FEE,
         AMOUNT-DEVICE_FEE, 0)

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