Symmetric decrypting returns a NULL value

Question

I have two databases . I copied all the data from one table and inserted into another database table . I have created symmetric key on second database but when i try to run the query as follow

 OPEN SYMMETRIC KEY SecureSymmetricKey DECRYPTION BY PASSWORD = N'StrongPassword';        
 select DecryptByKey(columname) as DocSSN from tablename
 CLOSE SYMMETRIC KEY SecureSymmetricKey; 

but allway return null value.

Answer 1

You need to follow the steps described in Create Identical Symmetric Keys on Two Servers. You will have to re-encrypt all the data on both servers/databases with a newly created symmetric key that uses the given known key material. Creating a key copy after the fact is not possible.

Sharing symmetric keys between servers/databases is a very bad practice. You should encrypt with different keys on each database.