SQL Server-如何将varchar转换为日期

Question

I have a table containing StartDate in the format dd/mm/yyyy and yyyy-mm-dd. I want to convert this varchar column to DATE type in the format DD/MM/YYYY.

I have tried the below.

select CONVERT(varchar(20),StartDate,103) AS [FormattedDate]

and

CONVERT(VARCHAR(20),(CAST([StartDate] AS DATE)),103)

I get the error -Conversion failed when converting date and/or time from character string.

Pls suggest.

Answer 1

SQL Server is actually quite good about figuring out formats for a date conversion with no formatting argument. However, it is going to assume MM/DD/YYYY for the second format and generate an error.

So, you can use try_convert() and coalesce() :

select coalesce(try_convert(date, startdate, 103),
                convert(date, startdate)
               )

Here is a SQL Fiddle.

Then, you should go into your data and fix the column. Here is one method:

update t
    set startdate = coalesce(try_convert(date, startdate, 103),
                             convert(date, startdate)
                            );

alter table t alter column startdate date;

You can add additional formatting for the result set by turning the date back into a string, using convert() .

Answer 2

To get YYYY-MM-DD use SELECT CONVERT(varchar, getdate(), 23)

To get MM/DD/YYYY use SELECT CONVERT(varchar, getdate(), 1)

For detailed explaination try this .

Answer 3

if you only have the date string in dd/mm/yyyy or yyyy-mm-dd

select case when substring(StartDate, 3, 1) = '/' 
            then convert(date, StartDate, 103)
            else convert(date, StartDate, 121)
            end

Answer 4

Here's an example that first tries to convert the VARCHAR from a 'yyyy-mm-dd' format to the 'dd/mm/yyyy' format.

If that doesn't work out, then it just assumes it's already in the 'dd/mm/yyyy' format.
And then defaults to the first 10 characters from the string.

declare @TestTable table (StartDate varchar(10), DateFormatUsed varchar(10));

insert into @TestTable (StartDate, DateFormatUsed) values
 (convert(varchar(10),GetDate() ,103), 'dd/mm/yyyy')
,(convert(varchar(10),GetDate(), 20),  'yyyy-mm-dd')
;

select t.*,
coalesce(convert(varchar(10), try_convert(date,StartDate,20),103), left(StartDate,10)) as [FormattedDate]
from @TestTable t;

But try_convert is only available since MS SQL Server 2012.
For MS SQL Server 2008 we can use a CASE WHEN with a LIKE to check the format.

declare @TestTable table (StartDate varchar(30), DateFormatUsed varchar(30));

insert into @TestTable (StartDate, DateFormatUsed) values
 (convert(varchar(10),GetDate(), 103), 'dd/mm/yyyy')
,(convert(varchar(10),GetDate(), 20),  'yyyy-mm-dd')
,(convert(varchar(19),GetDate(), 20),  'yyyy-mm-dd hh:mi:ss')
;

select t.*,
(case 
 when StartDate like '[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]%' 
 then convert(varchar(10), convert(date, left(StartDate, 10), 20), 103) 
 else left(StartDate, 10) 
 end) as [FormattedDate]
from @TestTable t;