Convert varchar(20) date to date that looks like mm/dd/yyyy

Question

Good afternoon,
I am a total novice at SQL and have been tasked to change a varchar(20) date column (shows up as m/dd/yy) to show up as mm/dd/yyyy. I've looked at the previous posts, but being a novice, it's difficult to wade through the non-applicable stuff. The column
that needs to be changed is field2, but I'm not sure how to construct a statement to reflect my objective.
Thank you in advance. 

Answer 1

The sample uses a table variable. You can program it the same way for a table:
DECLARE @t TABLE (dt VARCHAR(20))
INSERT INTO @t
VALUES ('1/2/12')
,('3/10/10')
,('7/23/9')
,('aa')
,('12/31/99')
,('10/20/00')
,('10/20/01')
SET DATEFORMAT mdy
SELECT dt
,CONVERT(VARCHAR(10), CASE
WHEN isdate(dt) = 0
THEN NULL
ELSE cast(dt AS DATE)
END, 101) AS newDt
FROM @t;/******** TRY_CONVERT() method ************/
SELECT dt, newDt=CONVERT(varchar(10),TRY_CONVERT(date,dt),101) FROM @t;
GO
dt newDt
1/2/12 01/02/2012
3/10/10 03/10/2010
7/23/9 07/23/2009
aa NULL
12/31/99 12/31/1999
10/20/00 10/20/2000
10/20/01 10/20/2001
dt newDt
1/2/12 01/02/2012
3/10/10 03/10/2010
7/23/9 07/23/2009
aa NULL
12/31/99 12/31/1999
10/20/00 10/20/2000
10/20/01 10/20/2001
Starting with SQL Server 2012 you can use TRY_CONVERT().
You can start studying datetime functions here:
http://www.sqlusa.com/bestpractices/datetimeconversion/
Kalman Toth Database & OLAP Architect
T-SQL Scripts at sqlusa.com
New Book / Kindle: Exam 70-461 Bootcamp: Querying Microsoft SQL Server 2012