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While they share similar syntaxes, there are lots of differences between CQL and SQL. The reasons for these differences come mainly from the fact that Cassandra is dealing with distributed data and aims to prevent inefficient queries. One of the places where CQL differs a lot from SQL is the WHERE clause. The goal of this post is to describe what is supported by the CQL WHERE clause and the reasons why it differs from normal SQL.

Primary key columns

In Cassandra, two types of columns have a special role: the partition key columns and the clustering columns. Together, they will define your row primary key. The partition key columns are the first part of primary key and their role is to spread data evenly around the cluster. Rows will be spread around the cluster based on the hash of the partition keys. The clustering key columns are used to cluster the data of a partition, allowing a very efficient retrival of rows. Due to the differences in the role that they are playing, partition key, clustering and normal columns support different sets of restrictions within the WHERE clause. Futhermore, those sets of restrictions differ depending of the type of query: SELECT, UPDATE or DELETE.

WHERE clause restrictions for SELECT statements

Partition keys restrictions

The partition key columns support only two operators: = and IN

IN restriction

Prior to 2.2, the IN restrictions could only be applied to the last column of the partition key. So, for example, if your table was:

CREATE TABLE numberOfRequests ( cluster text, date text, time text, numberOfRequests int, PRIMARY KEY ((cluster, date), time) )

In 2.1, you could only use an IN operator on the date column. In 2.2 you can use the IN operator on any partition key column. By consequence, a query like:

SELECT * FROM numberOfRequests WHERE cluster IN ('cluster1', 'cluster2') AND date = '2015-05-06' AND time >= '12:00' AND time <= '14:00';

is valid since 2.2 but is invalid in the prior releases. This change makes CQL more consistent but you should nevertheless be careful with IN restrictions on partition key columns. The nice post of Ryan Svihla will give you a clear explanation on why you should try to avoid them. Another change, introduced with 2.2, is that the results are not returned anymore in the order in which the partition key where specified in the IN clause. From 2.2 onward, the results are returned in the natural order of the column type and duplicates are ignored.

Unrestricted partition key columns

Cassandra will require that you either restrict all the partition key columns, or none of them unless the query can use a secondary index. This means that a query like:

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SELECT * FROM numberOfRequests WHERE cluster='cluster1' AND time ='12:00';

will be rejected as the date column is not restricted. The reason why is that Cassandra needs all the partition key columns to be able to compute the hash that will allow it to locate the nodes containing the partition. If no restrictions are specified on the partition keys but some are specified on the clustering keys, Cassandra will require ALLOW FILTERING to be added to the query. For more information on ALLOW FILTERING you should look at ALLOW FILTERING explained.

>, >=, <= and < operators

Cassandra distributes the partition accross the nodes using the selected partitioner. As only the ByteOrderedPartitioner keeps an ordered distribution of data Cassandra does not support >, >=, <= and < operator directly on the partition key. Instead, it allows you to use the >, >=, <= and < operator on the partition key through the use of the token function.

SELECT * FROM numberOfRequests WHERE token(cluster, date) > token('cluster1', '2015-06-03') AND token(cluster, date) <= token('cluster1', '2015-06-05') AND time = '12:00';

If you use a ByteOrderedPartitioner, you will then be able to perform some range queries over multiple partitions. You should nevertheless be careful. Using a ByteOrderedPartitioner is not recommended as it can result in unbalanced clusters.

Clustering column restrictions

Clustering columns support the =, IN, >, >=, <=, <, CONTAINS and CONTAINS KEY operators in single-column restrictions and the =, IN, >, >=, <= and < operators in multi-column restrictions.

Unrestricted clustering columns

The role of clustering columns is to cluster data within a partition. If you have the following table:

CREATE TABLE numberOfRequests ( cluster text, date text, datacenter text, hour int, minute int, numberOfRequests int, PRIMARY KEY ((cluster, date), datacenter, hour, minute))

The data will be stored per partition in the following way:

{datacenter: US_WEST_COAST {hour: 0 {minute: 0 {numberOfRequests: 130}} {minute: 1 {numberOfRequests: 125}} … {minute: 59 {numberOfRequests: 97}}} {hour: 1 {minute: 0 …

You can see that in order to retrieve data in an efficient way without a secondary index, you need to know all the clustering key columns for you selection. So, if you execute:

SELECT * FROM numberOfRequests WHERE cluster = ‘cluster1’ AND date = ‘2015-06-05’ AND datacenter = 'US_WEST_COAST' AND hour = 14 AND minute = 00;

Cassandra will find the data efficiently but if you execute:

SELECT * FROM numberOfRequests WHERE cluster = ‘cluster1’ AND date = ‘2015-06-05’ AND hour = 14 AND minute = 0;

Cassandra will reject the query as it has to scan the entire partition to find the requested data, which is inefficient.

IN restrictions

Prior to 2.2 the IN restrictions on clustering columns are only allowed on the last clustering column. In 2.2, the IN restriction can be used on any column and the following query will work:

SELECT * FROM numberOfRequests WHERE cluster = ‘cluster1’ AND date = ‘2015-06-05’ AND datacenter = 'US_WEST_COAST' AND hour IN (14, 15) AND minute = 0;

The same set of data can be retrieved prior to 2.2 by using a multi-column IN restriction

SELECT * FROM numberOfRequests WHERE cluster = ‘cluster1’ AND date = ‘2015-06-05’ AND datacenter = 'US_WEST_COAST' AND (hour, minute) IN ((14, 0), (15, 0));

In 2.2, multi-column IN restrictions can be applied to any set of clustering columns.

SELECT * FROM numberOfRequests WHERE cluster = ‘cluster1’ AND date = ‘2015-06-05’ AND (datacentre, hour) IN (('US_WEST_COAST', 14), (‘US_EAST_COAST’, 17)) AND minute = 0;

Prior to 2.2, multi-column IN restrictions can only be applied to the last set of clustering columns being restricted. By consequence, the previous query is invalid in 2.1. But following query is a perfectly valid one.

SELECT * FROM numberOfRequests WHERE cluster = ‘cluster1’ AND date = ‘2015-06-05’ AND datacenter = 'US_WEST_COAST' AND (hour) IN ((14), (15));

>, >=, <= and < restrictions

Single column slice restrictions are allowed only on the last clustering column being restricted. Therefore, the following queries are valid:

SELECT * FROM numberOfRequests WHERE cluster = ‘cluster1’ AND date = ‘2015-06-05’ AND datacenter = 'US_WEST_COAST' AND hour= 12 AND minute >= 0 AND minute <= 30; SELECT * FROM numberOfRequests WHERE cluster = ‘cluster1’ AND date = ‘2015-06-05’ AND data

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