I have a shell script which contain database connection and runs a big query.
When I run the script on command promp, the desired output is getting spooled taking some time.
But when I setup a cronjob for that script, the spool file is not getting generated.

What could be the problem?

Thanks in advance

Almost always, the first and last place to look for why a cron job fails is environment variables. The user is not logging in and executing .profile or .bashrc

Check that out.

After my investingation, I observed that the cronjob is not failing, but the database connection inside the script is failing.
Any idea what could be the problem?

you have to give us more details to help you.

What is the db that you are trying to connect to ?

Connection could be failing due to 'n' number of reasons :
>>database bounce, even after receiving a proper connection id
>>apart from password authentication, validation with the name of the script that is trying to establish connection to the database

what is the return status of the command that fails establishing connection with the db ?