Question

我正在比较一些日期和时间，发现我不理解Firebird Date Literals和their conversions，我认为是这样。

请参阅此查询：

SELECT 
    iif('2016-01-25' <= '2016-01-25', 1, 0) AS TSCASE,
    iif('2016-01-25 00:00:00.000' <= '2016-01-25', 1, 0) AS TSCASE1,
    iif('2016-01-25 00:00:00.000' <= cast('2016-01-25' as timestamp), 1, 0) AS TSCASE2,
    iif(cast('2016-01-25 00:00:00.000' as timestamp) <= '2016-01-25', 1, 0) AS TSCASE3,
    iif('2016-01-25 00:00:00.000' <= '2016-01-25 00:00:00.000', 1, 0) AS TSCASE4,
    iif('2016-01-25 00:00:00.000' <= '2016-01-25 23:59:59.999', 1, 0) AS TSCASE5,
    iif('2016-01-25 00:00:00.000' <= '2016-01-25  23:59:59.999', 1, 0) AS TSCASE6,
    iif('2016-01-25 00:00:00.000' <= cast('2016-01-25  23:59:59.999' as timestamp), 1, 0) AS TSCASE7
FROM RDB$DATABASE

我期望每列结果都是1.但结果是这样：

TSCASE  TSCASE1 TSCASE2 TSCASE3 TSCASE4 TSCASE5 TSCASE6 TSCASE7
       1      0       1       1       1       1       0       1

为什么'2016-01-25 00:00:00.000' <= '2016-01-25'是假的，但是当我对cast执行TIMESTAMP时，这是真的吗？

为什么'2016-01-25 00:00:00.000' <= '2016-01-25 23:59:59.999'和'2016-01-25 00:00:00.000' <= cast('2016-01-25 23:59:59.999' as timestamp)为真，但'2016-01-25 00:00:00.000' <= '2016-01-25 23:59:59.999'为假？

请注意最后两个表达式中日期和时间之间的额外空间。我确实认为额外的空间不会改变结果，你可以从first link中看到这句话：

日期文字中的空格

元素之间可以出现空格或制表符。日期部分必须与时间部分分开至少一个空格。

Answer 1

问题是您没有在大多数表达式中比较时间戳和日期。相反，您正在比较字符串（CHAR）文字。

回答您的具体问题：

'2016-01-25 00:00:00.000' <= '2016-01-25'为false，因为字符串排序2016-01-25出现在2016-01-25 00:00:00.000之前
'2016-01-25 00:00:00.000' <= '2016-01-25 23:59:59.999'为false，因为2016-01-25 23:59:59.999在时间部分之前有第二个空格，而在字符串排序中，空格位于0之前。

要使用日期或时间戳文字，您需要使用DATE或TIMESTAMP显式地为字符串文字添加前缀，或者您需要使用强制转换，否则它只是一个字符串。将您的查询更改为下面的查询，您就可以获得预期的值：

SELECT 
    iif(DATE'2016-01-25' <= DATE'2016-01-25', 1, 0) AS TSCASE,
    iif(TIMESTAMP'2016-01-25 00:00:00.000' <= DATE'2016-01-25', 1, 0) AS TSCASE1,
    iif(TIMESTAMP'2016-01-25 00:00:00.000' <= cast('2016-01-25' as timestamp), 1, 0) AS TSCASE2,
    iif(cast('2016-01-25 00:00:00.000' as timestamp) <= DATE'2016-01-25', 1, 0) AS TSCASE3,
    iif(TIMESTAMP'2016-01-25 00:00:00.000' <= TIMESTAMP'2016-01-25 00:00:00.000', 1, 0) AS TSCASE4,
    iif(TIMESTAMP'2016-01-25 00:00:00.000' <= TIMESTAMP'2016-01-25 23:59:59.999', 1, 0) AS TSCASE5,
    iif(TIMESTAMP'2016-01-25 00:00:00.000' <= TIMESTAMP'2016-01-25  23:59:59.999', 1, 0) AS TSCASE6,
    iif(TIMESTAMP'2016-01-25 00:00:00.000' <= cast('2016-01-25  23:59:59.999' as timestamp), 1, 0) AS TSCASE7
FROM RDB$DATABASE

为什么比较Firebird中的Timestamp和date导致奇怪的值？

1 个答案: