如何在SQL Server中将“ DD / MM / YYYY”或“ YYYY-MM-DD”中的字符串转换为日期？

Question

I've got a string here which needs to be converted into date but the problem is that it could either be in 'DD/MM/YYYY' or 'YYYY-MM-DD' format.

I've already tried convert which only works for one of the two formats but not both:

declare @string nvarchar(255) = '2019-05-21'

declare @table table (date date)

insert into @table
select convert(date, @string, 111) as date

select * from @table


declare @string nvarchar(255) = '21/05/2019'

declare @table table (date date)

insert into @table
select convert(date, @string, 103) as date

select * from @table

Both of the above solutions result in an error is I use the other format.

Is there a way to get a string converted to date regardless of what format it is in?

Answer 1

Use try_convert() :

insert into @table
    select coalesce(try_convert(date, @string, 111),
                    try_convert(date, @string, 103)
                   ) as date

try_convert() returns NULL if the conversion fails. In that case, the conversion will move on to the next pattern. With coalesce() , you can have as many different formats as you like.

Answer 2

You can use TRY_PARSE or PARSE to parse the date literal using a specific culture.

The second format YYYY-MM-DD is an unambiguous date format for the "new" date types like date and datetime2 . It's not affected by the DATEFORMAT setting like datetime .

This means you only need to find one culture that can handle the first format. All of the following queries will return the same value :

select parse('21/05/2019' as date using 'en-GB')
-----
2019-05-21

select parse('2019-05-21' as date using 'en-GB')
-----
2019-05-21

select try_parse('21/05/2019' as date using 'en-GB')
-----
2019-05-21

select try_parse('2019-05-21' as date using 'en-GB')
-----
2019-05-21

Answer 3

If you are on SQL 2012 and above, you can use the FORMAT function.

The signature of this function is - FORMAT (value,format[,culture])

Example: SELECT FORMAT (getdate(), 'dd-MM-yyyy') as date and in your case SELECT FORMAT(CAST(<str_value> as DATE), 'yyyy-mm-dd')