Keep null as empty and convert nvarchar to date - sql-server

Question

expiredDate nvarchar() column contains data like 22-12-2022 00:00:00 and null.
SELECT ISNULL(CONVERT(Date, expiredDate), '')
FROM tablename
Result for null is default date 1900-01-01 - I want it to be blank or empty.
As the column is getting converted to datetime null is converted to the default date.
Required date should be in yyyy-MM-dd format and null as empty

Answer 1

You may use CONVERT here on the raw date to generate a string, and then COALESCE missing null values into empty string:
SELECT expiredDate,
COALESCE(CONVERT(varchar(10), expiredDate, 120), '') AS expiredDateText
FROM yourTable;

Answer 2

You can try below query.
select case when ISDATE(expiredDate) = 0 then '' else convert(varchar,convert(date,expiredDate)) end as YourColumnName
The ISDATE() function checks whether the input data is a valid date or not. This includes NULL too. It will return Zero (0) if the data is not a date.
If you culture is en-us then by default you will get the date in YYYY-MM-DD format.
You can also use Format function in SQL Server to get the date in your expected format.
More information about format function can be found at below link.
https://learn.microsoft.com/en-us/sql/t-sql/functions/format-transact-sql?view=sql-server-ver15

Answer 3

You've have stumbled upon one of the odd(annoying) quirks in SQL Server where blank string becomes 1900-01-01 when cast to DATE. Your CONVERT() sets the column type to DATE, and then when you do ISNULL, it implicitly casts empty string to DATE. Why it returns 1900-01-01 instead of an error, I have no idea
Example of Empty String to Date Odd Behavior
SELECT CAST('' AS DATE)
--Returns 1900-01-01
Parse Date to Correct DATE data type and Then Format
How I'd recommend this is first converting your NVARCHAR column to the correct DATE data type. Using TRY_CAST allows any invalid dates to not cause the whole query to fail, instead will return NULL. FORMAT() then allows you to format the DATE anyway you like, and ISNULL will change NULL to empty string
DECLARE #YourTable TABLE (ID INT IDENTITY(1,1),expiredDate nvarchar(20))
INSERT INTO #YourTable
VALUES ('22-12-2022 00:00:00'),('31-12-2022 00:00:00'),('02-02-2022 00:00:00'),(NULL)
,('40-02-2022 00:00:00') /*Bad date for testing purposes*/
SELECT *,FormattedExpiredDate = ISNULL(FORMAT(B.ExpiredDateWithCorrectDateType,'yyyy-MM-dd'),'')
FROM #YourTable AS A
/*Format to YYYY-MM-DD (culture agnostic date format) and then use TRY_CAST(). If date is invalid, will return NULL*/
CROSS APPLY (SELECT ExpiredDateWithCorrectDateType = TRY_CAST(SUBSTRING(expiredDate,7,4) + SUBSTRING(expiredDate,4,2) + SUBSTRING(expiredDate,1,2) AS DATE)) AS B
Results:
ID
expiredDate
ExpiredDateWithCorrectDateType
FormattedExpiredDate
1
22-12-2022 00:00:00
2022-12-22
2022-12-22
2
31-12-2022 00:00:00
2022-12-31
2022-12-31
3
02-02-2022 00:00:00
2022-02-02
2022-02-02
4
NULL
NULL
5
40-02-2022 00:00:00
NULL