PRIMARY KEY (column_list)
Uniquely identifies rows, determines storage partitions, and orders data (clustering columns) within a partition.
A NULL value cannot be inserted into a PRIMARY KEY column.
This restriction applies to both partition keys and clustering columns.
- column_list
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Defines a partition and clustering columns, which affects how the data in stored. - 
Compound primary key: the first column is the partition key, and the additional columns are clustering keys. Syntax: PRIMARY KEY (<partition_column_name>, <clustering_column_name> [, ...])
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Composite partition key: Multiple columns in the partition key. Enclose the partition key columns in parentheses. Syntax: PRIMARY KEY ((<partition_column_name>[, ...]),<clustering_column_name> [, ...])
 
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