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# Join key
A set of fields that identify the records to include in a blended data source.
In a blended data source, each component data source must share one or more dimensions, known as a join key (or just a key, for short). Blended data sources include all the records from the leftmost data source in the Blend Data panel, and the matched records from the data sources to the right. In SQL terminology, this is known as a left outer join.
### Related resources
* [About data blending](broken://pages/-Mg-eddkiH4GsFKtbWPG)
* [Left outer join (Wikipedia)](https://en.wikipedia.org/wiki/Join_\(SQL\)#Left_outer_join)
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