Convert NULL to empty string - Conversion failed when converting from a character string to uniqueidentifier

SELECT Id   'PatientId',
       ISNULL(CONVERT(varchar(50),ParentId),'')  'ParentId'
FROM Patients

ISNULL always tries to return a result that has the same data type as the type of its first argument. So, if you want the result to be a string (varchar), you'd best make sure that's the type of the first argument.

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COALESCE is usually a better function to use than ISNULL, since it considers all argument data types and applies appropriate precedence rules to determine the final resulting data type. Unfortunately, in this case, uniqueidentifier has higher precedence than varchar, so that doesn't help.

(It's also generally preferred because it extends to more than two arguments)

Select ID, IsNull(Cast(ParentID as varchar(max)),'') from Patients

This is needed because field ParentID is not varchar/nvarchar type. This will do the trick:

Select ID, IsNull(ParentID,'') from Patients

You need to CAST the ParentId as an nvarchar, so that the output is always the same data type.

SELECT Id   'PatientId',
       ISNULL(CAST(ParentId as nvarchar(100)),'')  'ParentId'
FROM Patients