Robin Schumacher

This is an excerpt from the DataStax whitepaper "Data Modeling in Apache Cassandra™;"&nbsp;which delves into how to choose the right data model for your Apache Cassandra™ application in 5 easy steps. <a href="https://www.datastax.com/resources/whitepaper/data-modeling-apache-cassandra">Click here</a> to download the full whitepaper.

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Collections in Cassandra

When modeling the database, some developers might be tempted to store tags associated with videos in a separate table.&nbsp;When the list of anticipated tags is small however, using a collection data type that stores tags inside the database record can be more efficient.&nbsp;This simplifies the database design and reduces the number of tables required.

The five collection data types in Cassandra are:

<ul>
	<li>Set – a group collection of unique values of the same data type.</li>
	<li>List – an ordered collection of non-unique values of the same data type.</li>
	<li>Map – a set of key-value pairs, where keys are unique, and both keys and values have associated data types.</li>
	<li>Tuple – a fixed length list of non-unique values of different data types.</li>
	<li>Nested collection – a collection (i.e., set, list, map, or tuple) that is nested inside of another collection.</li>
</ul>

When defining a collection, the user needs to provide a data type for its elements.&nbsp;A simplified version of our videos table is provided below for illustration.

A sample row in this table may look as shown:

<img alt="CREATE TABLE" data-entity-type="file" data-entity-uuid="2c445409-3576-4b4f-871c-de618d2f1758" src="https://www.datastax.com/sites/default/files/inline-images/CM2019236_-_Data_Modeling_in_Apache_Cassandra_%E2%84%A2_White_Paper-4_pdf__page_13_of_19_.jpg" />

<img alt="videoid" data-entity-type="file" data-entity-uuid="136f0a7d-143a-42d9-b6a2-f142bd943441" src="https://www.datastax.com/sites/default/files/inline-images/CM2019236_-_Data_Modeling_in_Apache_Cassandra_%E2%84%A2_White_Paper-4_pdf__page_131_of_19_.jpg" />

CQL provides convenient syntax to insert, update, or delete items in collections.&nbsp;For example, a user can update the record for “My Funny Cat Video” and add a tag “wet cat” as shown:

<img alt="UPDATE videos" data-entity-type="file" data-entity-uuid="c058b924-88a9-4d12-8840-5a55c9c29b96" src="https://www.datastax.com/sites/default/files/inline-images/CM2019236_1-_Data_Modeling_in_Apache_Cassandra_%E2%84%A2_White_Paper-4_pdf__page_13_of_19_.jpg" />

User-defined data types

Another data type in Cassandra that provides flexibility is a user-defined type (UDT).&nbsp;UDTs can attach multiple data fields—each named and typed—to a single column.

Let’s assume that the designers of KillrVideo decide to store an optional mailing address for each user.&nbsp;Rather than add multiple address-related fields, an address type can be created and leveraged across multiple Cassandra tables.

<img alt="CREATE table" data-entity-type="file" data-entity-uuid="e861738d-a425-4058-87cd-6f250521490b" src="https://www.datastax.com/sites/default/files/inline-images/CM20191236_-_Data_Modeling_in_Apache_Cassandra_%E2%84%A2_White_Paper-4_pdf__page_13_of_19_.jpg" />

The user-defined address type can now be included in the users table as shown.&nbsp;The frozen keyword is required to use a UDT inside of a collection.&nbsp;It forces Cassandra to treat the address as a single value.&nbsp;Individual elements of a frozen address cannot be updated individually; rather, the entire address must be overwritten.

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Thanks for reading this&nbsp;excerpt from the DataStax whitepaper "Data Modeling in Apache Cassandra™;"&nbsp;tune in next week when we release another excerpt or&nbsp;<a href="https://www.datastax.com/resources/whitepaper/data-modeling-apache-cassandra">click here</a> to download the full asset.

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Collections in Cassandra

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