Difference between partition key, composite key and clustering key in Cassandra?

There is a lot of confusion around this, I will try to make it as simple as possible.

The primary key is a general concept to indicate one or more columns used to retrieve data from a Table.

The primary key may be SIMPLE and even declared inline:

 create table stackoverflow_simple (
      key text PRIMARY KEY,
      data text      
  );

That means that it is made by a single column.

But the primary key can also be COMPOSITE (aka COMPOUND), generated from more columns.

 create table stackoverflow_composite (
      key_part_one text,
      key_part_two int,
      data text,
      PRIMARY KEY(key_part_one, key_part_two)      
  );

In a situation of COMPOSITE primary key, the “first part” of the key is called PARTITION KEY (in this example key_part_one is the partition key) and the second part of the key is the CLUSTERING KEY (in this example key_part_two)

Please note that both partition and clustering key can be made by more columns, here’s how:

 create table stackoverflow_multiple (
      k_part_one text,
      k_part_two int,
      k_clust_one text,
      k_clust_two int,
      k_clust_three uuid,
      data text,
      PRIMARY KEY((k_part_one, k_part_two), k_clust_one, k_clust_two, k_clust_three)      
  );

Behind these names …

The Partition Key is responsible for data distribution across your nodes.
The Clustering Key is responsible for data sorting within the partition.
The Primary Key is equivalent to the Partition Key in a single-field-key table (i.e. Simple).
The Composite/Compound Key is just any multiple-column key

Further usage information: DATASTAX DOCUMENTATION

Small usage and content examples
***SIMPLE*** KEY:

insert into stackoverflow_simple (key, data) VALUES ('han', 'solo');
select * from stackoverflow_simple where key='han';

table content

key | data
----+------
han | solo

COMPOSITE/COMPOUND KEY can retrieve “wide rows” (i.e. you can query by just the partition key, even if you have clustering keys defined)

insert into stackoverflow_composite (key_part_one, key_part_two, data) VALUES ('ronaldo', 9, 'football player');
insert into stackoverflow_composite (key_part_one, key_part_two, data) VALUES ('ronaldo', 10, 'ex-football player');
select * from stackoverflow_composite where key_part_one="ronaldo";

table content

 key_part_one | key_part_two | data
--------------+--------------+--------------------
      ronaldo |            9 |    football player
      ronaldo |           10 | ex-football player

But you can query with all keys (both partition and clustering) …

select * from stackoverflow_composite 
   where key_part_one="ronaldo" and key_part_two  = 10;

query output

 key_part_one | key_part_two | data
--------------+--------------+--------------------
      ronaldo |           10 | ex-football player

Important note: the partition key is the minimum-specifier needed to perform a query using a where clause.
If you have a composite partition key, like the following

eg: PRIMARY KEY((col1, col2), col10, col4))

You can perform query only by passing at least both col1 and col2, these are the 2 columns that define the partition key. The “general” rule to make query is you must pass at least all partition key columns, then you can add optionally each clustering key in the order they’re set.

so, the valid queries are (excluding secondary indexes)

col1 and col2
col1 and col2 and col10
col1 and col2 and col10 and col 4

Invalid:

col1 and col2 and col4
anything that does not contain both col1 and col2

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