SQL Server - How to convert varchar to date - sql

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

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 2

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 3

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 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;