SELECT

Specifying columns

The columns referenced in the SELECT clause must exist in the target table.

Columns in big data applications duplicate values. Use the DISTINCT keyword to return only distinct (different) values of partition keys.

The SELECT statement supports functions that perform calculations on the columns being returned. For details, see Retrieving aggregate values

Using a column alias

When your selection list includes functions or other complex expressions, use aliases to make the output more readable. This example applies aliases to the dateOf(created_at) and blobAsText(content) functions:

SELECT event_id, 
  dateOf(created_at) AS creation_date,
  blobAsText(content) AS content 
  FROM timeline;

The output labels these columns with more understandable names:

event_id                | creation_date            | content
-------------------------+--------------------------+----------------
 550e8400-e29b-41d4-a716 | 2013-07-26 10:44:33+0200 | Some stuff

Specifying the source table using FROM

The FROM clause specifies the table to query. You may want to precede the table name with the name of the keyspace followed by a period (.). If you do not specify a keyspace, Cassandra queries the current keyspace.

The following example SELECT statement returns the number of rows in the IndexInfo table in the system keyspace:

SELECT COUNT(*) 
FROM system.IndexInfo;

Controlling the number of rows returned using LIMIT

The LIMIT option sets the maximum number of rows that the query returns:

SELECT lastname 
FROM cycling.cyclist_name 
LIMIT 50000;

Even if the query matches 105,291 rows, Cassandra only returns the first 50,000.

The cqlsh shell has a default row limit of 10,000. The Cassandra server and native protocol do not limit the number of returned rows, but they apply a timeout to prevent malformed queries from causing system instability.

Filtering data using WHERE

The WHERE clause introduces one or more relations that filter the rows returned by SELECT.

The column specifications

Filtering on the partition key

CREATE TABLE cycling.cyclist_career_teams ( id UUID PRIMARY KEY, lastname text, teams set<text> );

SELECT id, lastname, teams 
FROM cycling.cyclist_career_teams 
WHERE id=5b6962dd-3f90-4c93-8f61-eabfa4a803e2;

Restriction: a relation that references the partition key can only use an equality operator — = or IN. For more details about the IN operator, see Examples below.

Filtering on a clustering column

Use a relation on a clustering column only if it is preceded by relations that reference all the elements of the partition key.

Example:

CREATE TABLE cycling.cyclist_points (
   id UUID, 
   firstname text, 
   lastname text, 
   race_title text, 
   race_points int, 
   PRIMARY KEY (id, race_points ));

SELECT sum(race_points) 
FROM cycling.cyclist_points 
WHERE id=e3b19ec4-774a-4d1c-9e5a-decec1e30aac 
      AND race_points > 7;

Output:

 system.sum(race_points)
-------------------------
                     195

(1 rows)

Filtering on indexed columns

A WHERE clause in a SELECT on an indexed table must include at least one equality relation to the indexed column. For details, see Indexing a column.

Using the IN operator

Use IN, an equals condition operator, to list multiple possible values for a column. Thjs example selects two columns, first_name and last_name, from three rows having employee ids (primary key) 105, 107, or 104:

SELECT first_name, last_name 
FROM emp 
WHERE empID IN (105, 107, 104);

The list can consist of a range of column values separated by commas.

Using IN to filter on a compound or composite primary key

CREATE TABLE parts (
   part_type text, 
   part_name text, 
   part_num int, 
   part_year text, 
   serial_num text, 
   PRIMARY KEY ((part_type, part_name), part_num, part_year));

SELECT * 
FROM parts 
WHERE part_type='alloy' AND part_name='hubcap' 
AND part_num=1249 AND part_year IN ('2010', '2015');

SELECT * 
FROM parts 
WHERE part_num=123456 AND part_year IN ('2010', '2015') 
ALLOW FILTERING;

CQL supports an empty list of values in the IN clause, useful in Java Driver applications when passing empty arrays as arguments for the IN clause.

When not to use IN

Under most conditions, using IN in relations on the partition key is not recommended. To process a list of values, the SELECT may have to query many nodes, which degrades performance. For example, consider a single local datacenter cluster with 30 nodes, a replication factor of 3, and a consistency level of LOCAL_QUORUM. A query on a single partition key query goes out to two nodes. But if the SELECT uses the IN condition, the operation can involve more nodes — up to 20, depending on where the keys fall in the token range.

Using IN for clustering columns is safer. See Cassandra Query Patterns: Not using the “in” query for multiple partitions for additional logic about using IN.

Filtering on collections

Your query can retrieve a collection in its entirety. It can also index the collection column, and then use the CONTAINS condition in the WHERE clause to filter the data for a particular value in the collection, or use CONTAINS KEY to filter by key. This example features a collection of tags in the playlists table. The query can index the tags, then filter on 'blues' in the tag set.

SELECT album, tags 
FROM playlists 
WHERE tags CONTAINS 'blues';

SELECT * 
FROM playlists 
WHERE venue 
CONTAINS 'The Fillmore';

SELECT * 
FROM playlists 
WHERE venue CONTAINS KEY '2014-09-22 22:00:00-0700';

Filtering a map's entries

CREATE INDEX blist_idx 
ON cycling.birthday_list (ENTRIES(blist));

SELECT * 
FROM cycling.birthday_list 
WHERE blist['age'] = '23';

Filtering a full frozen collection

CREATE INDEX rnumbers_idx 
ON cycling.race_starts (FULL(rnumbers));

SELECT * 
FROM cycling.race_starts 
WHERE rnumbers = [39,7,14];

Range relations

The TOKEN function may be used for range queries on the partition key.

Cassandra supports greater-than and less-than comparisons, but for a given partition key, the conditions on the clustering column are restricted to the filters that allow Cassandra to select a contiguous set of rows.

CREATE TABLE ruling_stewards (
  steward_name text,
  king text,
  reign_start int,
  event text,
  PRIMARY KEY (steward_name, king, reign_start)
); 

SELECT * FROM ruling_stewards
WHERE king = 'Brego'
  AND reign_start >= 2450
  AND reign_start < 2500 
ALLOW FILTERING;

 steward_name | king  | reign_start | event
--------------+-------+-------------+--------------------
      Boromir | Brego |        2477 |   Attacks continue
       Cirion | Brego |        2489 | Defeat of Balchoth
       
(2 rows)

To allow Cassandra to select a contiguous set of rows, the WHERE clause must apply an equality condition to the king component of the primary key. The ALLOW FILTERING clause is also required. ALLOW FILTERING provides the capability to query the clustering columns using any condition.

Bad Request: Cannot execute this query as it might involve data
filtering and thus may have unpredictable performance. If you want
to execute this query despite the performance unpredictability,
use ALLOW FILTERING.

Select * 
FROM ruling_stewards
WHERE king = 'none'
  AND reign_start >= 1500
  AND reign_start < 3000 
LIMIT 10 
ALLOW FILTERING;

Comparing clustering columns

SELECT * 
FROM ruling_stewards 
WHERE (steward_name, king) = ('Boromir', 'Brego');

The syntax used in the WHERE clause compares records of steward_name and king as a tuple against the Boromir, Brego tuple.

Using compound primary keys and sorting results

ORDER BY clauses can only work on a single column. That column must be the second column in a compound PRIMARY KEY. This also applies to tables with more than two column components in the primary key. Ordering can be done in ascending or descending order using the ASC or DESC keywords (default is ascending).

In the ORDER BY clause, refer to a column using the actual name, not an alias.

For example, set up the playlists table (which uses a compound primary key), and use this query to get information about a particular playlist, ordered by song_order. You do not need to include the ORDER BY column in the select expression.

SELECT * FROM playlists 
WHERE id = 62c36092-82a1-3a00-93d1-46196ee77204
ORDER BY song_order DESC 
LIMIT 50;

Output:

Or, create an index on playlist artists, and use this query to get titles of Fu Manchu songs on the playlist:

CREATE INDEX ON playlists(artist);

SELECT album, title 
FROM playlists 
WHERE artist = 'Fu Manchu';

Output:

Displaying rows from an unordered partitioner with the TOKEN function

Use the TOKEN function to display rows based on the hash value of the partition key, for a single partition table. Selecting a slice using TOKEN values will only work with clusters that use the ByteOrderedPartitioner.

For example, create this table:

CREATE TABLE cycling.last_3_days (
  race_name text, 
  year timestamp, 
  rank int, 
  cyclist_name text, 
  PRIMARY KEY (year, rank, cyclist_name)
);

After inserting data, SELECT using the TOKEN function to find the data using the partition key.

SELECT * 
FROM cycling.last_3_days 
WHERE TOKEN(year) < TOKEN('2015-05-26') 
  AND year IN ('2015-05-24','2015-05-25');

Computing aggregates

Cassandra provides standard built-in functions that return aggregate values to SELECT statements.

Using COUNT() to get the non-NULL value count for a column

A SELECT expression using COUNT(column_name) returns the number of non-NULL values in a column.

For example, count the number of last names in the cyclist_name table:

SELECT COUNT(lastname) 
FROM cycling.cyclist_name;

Getting the number of matching rows and aggregate values with COUNT()

A SELECT expression using COUNT(*) returns the number of rows that matched the query. Use COUNT(1) to get the same result. COUNT(*) or COUNT(1) can be used in conjunction with other aggregate functions or columns.

This example returns the number of rows in the users table:

SELECT COUNT(*) 
FROM users;

This example counts the number of rows and calculates the maximum value for points in the users table:

SELECT name, max(points), COUNT(*) 
FROM users; 

Getting maximum and minimum values in a column

SELECT MAX(points) 
FROM cycling.cyclist_category;

Output:

SELECT category, MIN(points) 
FROM cycling.cyclist_category 
WHERE category = 'GC';

Output:

Getting the sum or average of a column of numbers

Cassandra computes the sum or average of all values in a column when SUM or AVG is used in the SELECT statement:

Note: If any of the rows returned has a null value for the column referenced for AVG aggregation, Cassandra includes that row in the row count, but uses a zero value to calculate the average.

Note: The sum and avg functions do not work with text, uuid or date fields.

Retrieving the date/time a write occurred

The WRITETIME function applied to a column returns the date/time in microseconds at which the column was written to the database.

R\For example, to retrieve the date/time that a write occurred to the first_name column of the user whose last name is Jones:

SELECT WRITETIME (first_name) 
FROM users 
WHERE last_name = 'Jones';

 writetime(first_name)
-----------------------
 1353010594789000

The WRITETIME output in microseconds converts to November 15, 2012 at 12:16:34 GMT-8.

Retrieving the time-to-live of a column

INSERT INTO cycling.calendar (race_id, race_name, race_start_date, race_end_date) 
VALUES (200, 'placeholder', '2015-05-27', '2015-05-27') 
USING TTL;

UPDATE cycling.calendar 
USING TTL 300 
SET race_name = 'dummy' 
WHERE race_id = 200 
  AND race_start_date = '2015-05-27' 
  AND race_end_date = '2015-05-27';

SELECT TTL(race_name) 
FROM cycling.calendar 
WHERE race_id=200;

 ttl(race_name)
----------------
            276

(1 rows)

Retrieving values in the JSON format

For details, see Retrieval using JSON