Converting strings to date in SQL - sql

Question

I am using SQL Server 2017. I am trying to handle strings from a free format field and either convert them to a date in the format of "dd/mm/yyyy" or if they are not in this format then simply display the text verbatim.
I need this in a VIEW so can not use SET LANGUAGE. Sounds simple using Convert and IsDate but does not seem to work.
So for the snippet of code below (remember this will be in a view), I want to read the text and if the string converts to a date (ie. is in the format dd/mm/yyyy then run the convert to a date as I need it in date format for Excel to pick up (via Connect SQL Server database)), and if it does not convert to a date then display the text as it is.
create table dateTest1
(
idx int,
dateStringTest varchar(15)
);
insert into dateTest1 (idx, dateStringTest)
values (1, '13/01/2021'), (2, 'no');
select
case
when isdate(convert(datetime, dateStringTest, 103)) = 1
then convert(datetime, dateStringTest, 103)
else dateStringTest
end as dtres
from
dateTest1
--where idx = 1
Error:
Msg 241, Level 16, State 1, Line 15
Conversion failed when converting date and/or time from character string.
This error happens for idx = 2. Idx = 1 works ok.
Any assistance with this would be greatly appreciated. Thanks in advance.

Answer 1

You need to cast your resulting date to a varchar. A case expression can only return a single data type and the order-of-precedence means it is still trying to convert the varchar values to datetime
select
case when isdate( dateStringTest) = 1
then Cast(convert(datetime, dateStringTest, 103) as varchar(10))
else dateStringTest
end as dtres
from dateTest1
You can compact into a single statement (the same order of precedence applies)
select IsNull(Cast(Convert(datetime,Try_Cast(dateStringTest as date),103) as varchar(10)),dateStringTest)
from datetest1

Answer 2

Based on the syntax you are using, I assume you are using SQL Server. You should add the appropriate tag to your question.
The only way to do that in one column is to leave it as text.
Your use of ISDATE() is incorrect. If dateStringTest is not a valid date, CONVERT() on SQL Server will throw an error like:
The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.
That's why you get an error for idx=2
You say you "need it in date format for Excel to pick up". How Excel interprets it will probably depend on your locale settings. Assuming mm/dd/yyyy is valid in your locale, Excel probably already sees it as a date. But I assume that's not happening, so mm/dd/yyyy is not valid for your locale. For me, if I have this table in SQL...
create table dateTest1 (
id int identity(1,1) not null,
dateStringTest varchar(20)
)
insert dateTest1
values
('2020-01-01')
, ('2020-21-01')
, ('2020-01-21')
, ('1/1/2020')
, ('1/21/2020')
, ('21/1/2020')
, ('21/01/2020')
, ('other stuff')
...and query it from Excel, adding columns with the functions DATEVALUE, DAY, WEEKDAY, and YEAR, I get...
id
dateStringTest
DATEVALUE
DAY
WEEKDAY
YEAR
1
2020-01-01
43831
1
4
2020
2
2020-21-01
#VALUE!
#VALUE!
#VALUE!
#VALUE!
3
2020-01-21
43851
21
3
2020
4
1/1/2020
43831
1
4
2020
5
1/21/2020
43851
21
3
2020
6
21/1/2020
#VALUE!
#VALUE!
#VALUE!
#VALUE!
7
21/01/2020
#VALUE!
#VALUE!
#VALUE!
#VALUE!
8
other stuff
#VALUE!
#VALUE!
#VALUE!
#VALUE!
That seems to indicate that Excel is recognizing some of the values as dates.
What you need to do is try to convert the value to datetime, then if it fails report the original value. In both cases, output a string.
You should review the documentation for CONVERT().
Try this:
select dateStringTest
, coalesce(cast(try_convert(datetime, dateStringTest, 103) as varchar(20)), dateStringTest) as dtres1
, coalesce(convert(varchar(20), try_convert(datetime, dateStringTest, 103), 103), dateStringTest) as dtres2
, coalesce(convert(varchar(20), try_convert(datetime, dateStringTest, 103), 101), dateStringTest) as dtres3
from dateTest1
UPDATE
If you are considering only values matching the pattern dd/mm/yyyy as dates (so 17/01/2021 is a date and 17/1/2021 is not), this brute force method will work with SQL Server 2008 (compatibility level 100):
(Notice I updated my input, also, above.)
;
with a as (
select id
, dateStringTest
, SUBSTRING(dateStringTest, 7, 4) + '-' + SUBSTRING(dateStringTest, 4, 2) + '-' + SUBSTRING(dateStringTest, 1, 2) as converteDate
from dateTest1
)
select id
, cast(convert(datetime, dateStringTest, 103) as varchar(20)) as dtres
from a
where isdate(converteDate) = 1
union
select id
, dateStringTest as dtres
from a
where isdate(converteDate) = 0