Delete data from data store having some conditions to column similar to where clause in RDBMS

You will need to first fetch the primary key values from that table using queryEntity, and then delete the rows based on that key.

could u pls elaborate ?

What is your question? What did you try? What was the outcome? Any code snippets?

1. you can only delete from the database table using the identifiers (primary keys) of the rows you want to delete when using deleteFromDataStoreEntities 
2. you therefore first need to fetch those primary keys. To fetch them you need to, as Stefan described, run a query using a!queryEntity() and use the values you have described above as filters for the rows you want returned from the database

My question is rather simple, would I be able to delete the rows based on some condition (similar to Where clause in SQL) ?

Is their any way by which I can write an expression rule for truncating the appian db table ? Please help in this regards.

Yes, in the way I tried to explain.

For the truncate action, I suggest to use the Query Database Smart Service. It allows you to execute SQL statement directly on the database.

but I a using the appian cloud db. Would I be able to connect ?

Yes

Could u please give an example, how can I write expression rule for the same (connecting with the appian cloud db) so that I can fire sql queries ?

You will find all the required details in the Appian documentation.

docs.appian.com/.../Query_Database_Smart_Service.html