":: date"的应用是什么?在红移? [英] what is the application of "::date" in redshift?

问题描述

我在Amazon redshift的sql代码中看到比较两个日期时使用:: date的地方.我想知道这三行代码之间是否有任何区别:

I see somewhere in the sql code in Amazon redshift that ::date is used when comparing two dates. I am wondering is there any difference between these three lines of code:

start_date < '2016-01-01'
start_date < '2016-01-01'::date
start_date < date('2016-01-01')

推荐答案

在所有三种情况下结果都是相同的.

The result is the same in all three cases.

特别是:

• start_date < '2016-01-01'试图将date与varchar进行比较，但是Amazon Redshift足够聪明，可以将varchar转换为日期格式以进行比较.

• start_date < '2016-01-01' is trying to compare a date with a varchar, but Amazon Redshift is smart enough to convert the varchar into a date format for comparison purposes.

start_date < '2016-01-01'::date在两个date字段之间进行了适当的比较.这相当于date '2016-01-01'.

start_date < '2016-01-01'::date is doing a proper comparison between two date fields. This would be equivalent to date '2016-01-01'.

start_date < date('2016-01-01')似乎也在比较date字段，尽管该语法不在

start_date < date('2016-01-01') appears to also be comparing date fields, although that syntax isn't in the Date and Time Functions documentation.

使用::date的一个更有用的示例是比较两个时间戳，而您只希望比较日期，例如:

A more useful example of using ::date is when comparing two timestamps, and you only wish to compare the date, for example:

select end::date - start::date as days FROM table

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