Which database is best for OLTP and OLAP queries at the same time?

You cant say any one product is best for both OLTP and OLAP. Both are fundamentally different in design and architecture behind would also have an effect on how the system perform. For example, OLTP is more transaction oriented and is most optimised for data write operations whereas in the case OLAP systems its mostly data read and aggregation that takes place.

Thats why recently OLAP systems mostly go for columnar storage model which will cause data to store column wise and helps in aggregating data over columns.

also OLAP systems mostly do not enforce stringent constraints. Its up to ETL processes to ensure the relationships are kept intact.Also recently warehouse related products are based on MPP architecture rather than SMP architecture which OLTP systems are based on