# Querying the Database

## Query Functions
ChemRecon provides a number of convenience functions for querying the database. 
They are all imported into the `.chemrecon.query` module.

Querying entries requires providing both an indentifier and an identifier type.
For a list of identifier types, see [Identifier Types](/content/identifiertypes).

```{eval-rst}
.. automodule:: chemrecon.query
    :members:
    :imported-members:

```

## Advanced Queries with Query Handlers
Most common operations of ChemRecon can be performed using the built-in convenience functions described above.
These functions query the database using {class}`.QueryHandler` objects.
For more advanced functionality, {class}`.QueryHandler`s can be used directly.
After connecting, ChemRecon keeps a reference to the active query handler, which can be accessed using
{func}`.get_query_handler`.


```{eval-rst}
.. autoclass:: chemrecon.QueryHandler

```

Any operations which require modifying the database need to be called from a {class}`.PopulateQueryHandler`.
Note that this is only possible on self-hosted databases, not the public database.

```{eval-rst}
.. autoclass:: chemrecon.PopulateQueryHandler

```