Case when only executes ELSE statement - sql-server

Question

I have never seen this happen before, but when I SELECT a CASE WHEN statement in SQL Server, it executes the ELSE statement. How is this possible? Is it because it cannot handle different kind of value types in the same column?
SELECT CASE WHEN len(birthDate)=4 THEN birthDate
WHEN birthDate = '' THEN ''
ELSE CONVERT(datetime, birthDate)
END AS [birthDateConverted]
,[birthDate]
FROM BirthDayTable
The result looks like this:
birthDateConverted birthDate
1951-01-01 00:00:00.000 1951
1936-06-19 00:00:00.000 June 19, 1936
1948-03-11 00:00:00.000 March 11, 1948
NULL
I want to have the following:
birthDateConverted birthDate
1951 1951
1936-06-19 June 19, 1936
1948-03-11 March 11, 1948
NULL
And I also do not understand why I get NULL when I specify ''. But this part is not as important as the first part, as I would like to have only the year when only a year is specified.

Answer 1

Try this first and then implement the logic accordingly in your query.
Replace #birthDate variable with the input string that you desire.
There is one more check that you can perform using ISDATE() function which checks if the entered string is a valid date and you can change your logic accordingly if required.
More information about isdate() function can be found here
DECLARE #birthDate varchar(20)=''
SET #birthDate='March 11, 1948'
SELECT CASE WHEN len(#birthDate)=4 THEN CONVERT(varchar, #birthDate)
WHEN CONVERT(varchar,#birthDate) = '' THEN ''
ELSE CONVERT(varchar,CONVERT(date, #birthDate))
END AS [birthDateConverted]

Answer 2

NULL is not equal to '' so, you can do instead :
SELECT (CASE WHEN len(birthDate) = 4
THEN birthDate
WHEN birthDate IS NULL
THEN ''
ELSE CONVERT(datetime, birthDate)
END) AS [birthDateConverted],
[birthDate]
FROM BirthDayTable;
However, '' will implicit convert into datetime.

Answer 3

You are trying to store multiple types in one column which is not possible in SQL. So when the query executes on SQL Server it will type all the values on first read type.
In your case, your second conditions always fail due to comparing a null value with an empty string, in SQL you can achieve this through ISNULL(birthDate,'') = '' so when the value is null or empty it always returns true.
So you can't set an empty string or date string with DateTime value your requirement is invalid.
I think you are looking for this it will format DateTime values as string
SELECT (CASE WHEN ISDATE(birthDate) = 1
THEN CONVERT(varchar, CONVERT(datetime,birthDate), 110)
ELSE birthDate
END) AS [birthDateConverted], [birthDate]
FROM BirthDayTable;

Answer 4

I have tried the following, which works for my case. Hopefully someone else can benefit from it too
SELECT CASE WHEN len(birthDate)>4
THEN CONCAT(YEAR(birthDate), '-', RIGHT('00' + CONVERT(NVARCHAR(2), DATEPART(MONTH, (birthDate))),2), '-',
RIGHT('00' + CONVERT(NVARCHAR(2), DATEPART(DAY, (birthDate))),2))
WHEN len(birthDate)=4
THEN CONVERT(nvarchar(255), birthDate)
ELSE ''
END AS [birthDateConverted]
,[birthDate]
FROM BirthDayTable;

Answer 5

A CASE will only return 1 datatype.
In your SQL that would return a DATETIME because of that convert.
And since '1951' is then implicitly casted to a DATETIME, it returns '1951-01-01 00:00:00.000'.
So let the CASE return a VARCHAR.
If you're using MS SQL Server 2012 or higher, then you could use TRY_CONVERT to verify if the varchar can be converted to a date or datetime.
Because a TRY_CONVERT will return NULL when the conversion attempt fails.
Example snippet:
select *,
(case
when birthDate like '[0-9][0-9][0-9][0-9]' then birthDate
when birthDate like '%[0-9]%' and try_convert(date, birthDate) is not null
then convert(varchar(10), convert(date, birthDate), 20)
else ''
end) as birthDateConverted
from (values
(1,'1951'),
(2,'June 19, 1936'),
(3,'March 11, 1948'),
(4,''),
(5,null),
(6,'ABCD')
) t(id, birthDate);
Returns:
id birthDate birthDateConverted
-- -------------- ------------------
1 1951 1951
2 June 19, 1936 1936-06-19
3 March 11, 1948 1948-03-11
4
5 NULL
6 ABCD