NVARCHAR to DATE (SQL Server)

Question

I have table with the week_date stored as nvarchar(60) . When I tried to change that to date , I'm getting below error.

Records in the table are in yyyy-mm-dd format

WEEK_DATE
----------
2017-12-31
2018-01-01

Msg 241, Level 16, State 1, Line 1
Conversion failed when converting date and/or time from character string.

I have used the below to convert, but that throws an error

SET WEEK_DATE = CONVERT(NVARCHAR(260), WEEK_DATE, 120)

Thank you

Answer 1

Use try_convert() to find the bad values:

select datecol
from t
where try_convert(date, datecol) is null and datecol is not null;

You can use try_convert() in your code, but you should investigate the data problem.

EDIT:

To change the column to a date, first verify the format (so you don't get an error with the conversion):

update t
     set week_date = try_convert(date, week_date);

Don't worry about the format. It will use whatever the localization settings are. But happily, the resulting string will automatically be converted back to a date.

Then, alter the table:

alter table t
     alter week_date date;

You might want to use the first query to check that all dates strings are actually recognized.