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# Query Concepts

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In general, querying a data source means performing an action on or interacting with the contents of the data source. To do this, you must be able to perform the following:

- Define an action in a syntax native to the data source being queried

- Apply the action in a controlled fashion

- Manage the results returned by the action (if any)

Specific to TopLink, you must also consider how the query affects the TopLink cache. For more information, see "Queries and the Cache".

This section introduces query concepts unique to TopLink, including the following:

- Call

- DatabaseQuery

- Data-Level and Object-Level Queries

- Summary Queries

- Descriptor Query Manager

- TopLink Expressions

- Query Keys

- Query Languages