This has already been answered .Please refer the below links:

https://pdn.pega.com/community/pega-product-support/question/difference-between-obj-operations-and-rdb-operations-obj

https://pdn.pega.com/community/pega-product-support/question/diff-bw-obj-and-rdb-methods

There are couple of differences to be listed below:

1. RDB method can't fetch or update the data of Blob colum but Obj method can.

2. Deferred save option is available wth Obj Method but not with RDB.

3. In Terms of Performance RDB has better performance than Obj

1. First of all, Obj methods are pega pre-defined so we do not need to form a query for them, whereas RDB methods needs queries explicitly written on the connect SQL rule.

2. Obj methods are primarily used for querying on internal classes or pega related DB whereas RDB methods can be used to query upon internal as well as external DB.

3. Obj methods can only be queried upon a single class whereas RDB methods can be helpful in querying along with joins involving more than one class.

4. Obj methods are meant for transactional ease as they have the option of deferred save, rollback etc whereas RDB methods are performed immediately with auto-commit.