Clean a messy column and update table in SQL Server

Question

I have a table dbo.mytbl in SQL Server database where column sale_date is messy.

It contains something like None , empty string, not available , 2016 , etc - other than those date-like format 2019-12-05 .

It's currently of datatype nvarchar(max) .

I can clean up the column like below. All those not like dates are converted to NULL

SELECT 
    CASE 
       WHEN ISDATE(sale_date) = 1 
          THEN CONVERT(DATE, sale_date) 
          ELSE NULL 
    END AS sale_date
FROM dbo.mytbl

I want to update the table, clean and modify the sale_date column in the database, using alter table .

How can I achieve that?

ALTER TABLE dbo.mytbl
ALTER COLUMN ????????????;

Answer 1

Use try_convert() !

If you are not worried about the non-dates, you can do:

update mytbl
    set sale_date = try_convert(date, sale_date);

This will set all the non-dates to NULL . Then you can safely do:

alter table mytbl alter column sale_date date;