To replace the NULL values in your resultset, you could use the ISNULL() function. For example:
SELECT [ID], [Name] FROM [People]
would return something like
1 Chris
2 Null
3 Nancy
You could easily change the Null values to the value 'unknown' using the following:
SELECT [ID], ISNULL([Name], 'unknown') FROM [People]
The result would be
1 Chris
2 unknown
3 Nancy
More on this
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