How to convert a place holder viewed as "?" to Null?

Teradata Studio
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How to convert a place holder viewed as "?" to Null?

Hi Dear Teradata Users,

I am new to Teradata Studio and Aster database, beginner for SQL. I need help with something that might be at elementry level. 

I see a table that shows many "?" for unknow value (missing? Place holder?) Apparently it is no a NULL value currently becuase I used

WHERE COLUMN1 IS NOT NULL

and it gave me the error:

Preparing the parameterized resulted in an error message from the database.

[AsterData][JDBC](10060) Connection has been closed.

Do you wan tot continue processing the SQL with the unparameterized handler?

I also used NULLIF function like this

SELECT COLUMN1, NULLIF(COLUMN1, " ") FROM TABLE1

and it didn't work either.

I would like to convert the "?" to a NULL value. and I also would like to know how to handle it in the query.

I would appreciate any help or suggestoins.

Thank you,

Hanna