How to use NULL or empty string in SQL

If you need it in SELECT section can use like this.

SELECT ct.ID, 
       ISNULL(NULLIF(ct.LaunchDate, ''), null) [LaunchDate]
FROM   [dbo].[CustomerTable] ct

You can replace the null with your substitution value.

Select *
From Table
Where (col is null or col = '')

Or

Select *
From Table
Where IsNull(col, '') = ''