One call to `decryptbypassphrase` for decrypting all columns If I use ENCRYPTBYPASSPHRASE to encrypt the full table then how can I decrypt full table easily by using DECRYPTBYPASSPHRASE? 

Actually I have to use DECRYPTBYPASSPHRASE ('passphrase', column_name) for every column to decrypt it. So it becomes so lengthy command. If I want to decrypt all columns then I have to specify this statement for all columns. Instead of that, can I use DECRYPTBYPASSPHRASE in queries so that I can use it only once and it decrypts all the columns?
No. Scalar operators cannot magically transform themselves into relational operators. You use the ENCRYPTBYPASSPHRASE to encrypt a column (there is no way to 'encrypt a table') and you use DECRYPTBYPASSPHRASE to decrypt a column.

Using passphrases to encrypt the data directly is a very very poor choice. A proper key hierarchy is needed where data is encrypted with a set of symmetric keys and the symmetric keys are in turn encrypted with a certificate. Encrypting the data directly with a passphrase requires expensive size-of-data operation to rotate the keys.