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I am converting two Date columns to find the most recent one

SELECT ISNULL(CONVERT(varchar(10),REPLACE('10-07-2015','/','-'), 103),'01-01-1900'),
ISNULL(CONVERT(varchar(10),REPLACE('10/7/2015','/','-'), 103),'01-01-1900'),
 CASE 
 WHEN ISNULL(CONVERT(varchar,REPLACE('10-07-2015','/','-'), 103),'01-01-1900') = ISNULL(CONVERT(varchar,REPLACE('10/7/2015','/','-'), 103),'01-01-1900')
 THEN '10-07-2015'
 END

My issue is some dates missing leading Zero in Day or Month and comparison is giving false results. Is there a better way to handle this? Other issue is one column has date with '/' and other have with '-'

Currently case is only checking on '=' but will add more to get the most recent

You can just convert those 2 varchars to the DATE type, then compare them.

You can find the date/datetime styles here

For those DD/MM/YYYY datestamps the 103 style would fit.

And to calculate the most recent between them, just wrap it in a CASE.

Example snippet:

declare @T table (
 id int identity(1,1) primary key, 
 datestamp1 varchar(10), 
 datestamp2 varchar(10)
);

insert into @T (datestamp1, datestamp2) values 
 ('5/9/2018','17/9/2018')
,('9-10-2018','16-10-2018')
,('15-10-2018','13-10-2018')
;

SELECT *,
TRY_CONVERT(DATE, datestamp1, 103) as date1, 
TRY_CONVERT(DATE, datestamp2, 103) as date2,
CASE 
WHEN TRY_CONVERT(DATE, datestamp1, 103) >= TRY_CONVERT(DATE, datestamp2, 103) THEN datestamp1
WHEN TRY_CONVERT(DATE, datestamp2, 103) IS NULL THEN datestamp1
ELSE datestamp2
END AS MostRecentDatestamp
FROM @T;

Returns:

id datestamp1 datestamp2 date1 date2 MostRecentDatestamp
1 5/9/2018 17/9/2018 2018-09-05 2018-09-17 17/9/2018
2 9-10-2018 16-10-2018 2018-10-09 2018-10-16 16-10-2018
3 15-10-2018 13-10-2018 2018-10-15 2018-10-13 15-10-2018