Sorting and limiting results
Similar to SQL, CQL can SELECT data using simple or complex qualifiers. At its simplest, a query selects all data in a table. At its most complex, a query delineates which data to retrieve and display and even calculates new values based on user-defined functions. This section uses several example tables in the cycling keyspace.
For secondary indexing using Storage-Attached Indexing (SAI), see CREATE CUSTOM INDEX.
Controlling the number of rows returned using PER PARTITION LIMIT
The PER PARTITION LIMIT
option sets the maximum number of rows that the query returns from each partition.
Create a table that will sort data into more than one partition.
CREATE TABLE cycling.rank_by_year_and_name ( race_year int, race_name text, cyclist_name text, rank int, PRIMARY KEY ((race_year, race_name), rank) );
After inserting data, the table holds:
race_year | race_name | rank | cyclist_name -----------+--------------------------------------------+------+---------------------- 2014 | 4th Tour of Beijing | 1 | Phillippe GILBERT 2014 | 4th Tour of Beijing | 2 | Daniel MARTIN 2014 | 4th Tour of Beijing | 3 | Johan Esteban CHAVES 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 1 | Daniel MARTIN 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 2 | Johan Esteban CHAVES 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 3 | Benjamin PRADES 2015 | Giro d'Italia - Stage 11 - Forli > Imola | 1 | Ilnur ZAKARIN 2015 | Giro d'Italia - Stage 11 - Forli > Imola | 2 | Carlos BETANCUR 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 1 | Benjamin PRADES 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 2 | Adam PHELAN 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 3 | Thomas LEBAS
Now, to get the top two racers in every race year and race name, use the following command with PER PARTITION LIMIT 2
.
SELECT * FROM cycling.rank_by_year_and_name PER PARTITION LIMIT 2;
Output:
race_year | race_name | rank | cyclist_name -----------+--------------------------------------------+------+---------------------- 2014 | 4th Tour of Beijing | 1 | Phillippe GILBERT 2014 | 4th Tour of Beijing | 2 | Daniel MARTIN 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 1 | Daniel MARTIN 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 2 | Johan Esteban CHAVES 2015 | Giro d'Italia - Stage 11 - Forli > Imola | 1 | Ilnur ZAKARIN 2015 | Giro d'Italia - Stage 11 - Forli > Imola | 2 | Carlos BETANCUR 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 1 | Benjamin PRADES 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 2 | Adam PHELAN
Procedure
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Use a simple
SELECT
query to display all data from the cyclist category table.CREATE TABLE cycling.cyclist_category ( category text, points int, id UUID, lastname text, PRIMARY KEY (category, points)) WITH CLUSTERING ORDER BY (points DESC); SELECT * FROM cycling.cyclist_category;
category | points | id | lastname ---------------+--------+--------------------------------------+------------ One-day-races | 367 | 220844bf-4860-49d6-9a4b-6b5d3a79cbfb | TIRALONGO One-day-races | 198 | 6ab09bec-e68e-48d9-a5f8-97e6fb4c9b47 | KRUIJSWIJK Time-trial | 182 | 220844bf-4860-49d6-9a4b-6b5d3a79cbfb | TIRALONGO Time-trial | 3 | 6ab09bec-e68e-48d9-a5f8-97e6fb4c9b47 | KRUIJSWIJK Sprint | 39 | 6ab09bec-e68e-48d9-a5f8-97e6fb4c9b47 | KRUIJSWIJK Sprint | 0 | 220844bf-4860-49d6-9a4b-6b5d3a79cbfb | TIRALONGO GC | 1324 | 6ab09bec-e68e-48d9-a5f8-97e6fb4c9b47 | KRUIJSWIJK GC | 1269 | 220844bf-4860-49d6-9a4b-6b5d3a79cbfb | TIRALONGO
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The example below illustrates how to create a query that uses category as a filter.
SELECT * FROM cycling.cyclist_category WHERE category = 'Sprint';
category | points | id | lastname ----------+--------+--------------------------------------+------------ Sprint | 39 | 6ab09bec-e68e-48d9-a5f8-97e6fb4c9b47 | KRUIJSWIJK Sprint | 0 | 220844bf-4860-49d6-9a4b-6b5d3a79cbfb | TIRALONGO
The database rejects this query if category is not a partition key or clustering column, because the query cannot locate the data on disk. Queries require a sequential retrieval across the entire cyclist_category table. In DataStax Enterprise, this is a crucial concept to grasp; scanning all data across all nodes is prohibitively slow and thus blocked from execution. The use of partition key and clustering columns in a
WHERE
clause must result in the selection of a contiguous set of rows.You can filter queries using secondary indexes, see Building and maintaining indexes. A query based on lastname can result in satisfactory results if the lastname column is indexed.
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Clustering columns can be defined in
WHERE
clauses ifALLOW FILTERING
is also used even if a secondary index is not created. The table definition is given and then theSELECT
command. Note thatrace_start_date
is a clustering column that has no secondary index.CREATE TABLE cycling.calendar ( race_id int, race_name text, race_start_date timestamp, race_end_date timestamp, PRIMARY KEY (race_id, race_start_date, race_end_date) );
SELECT * FROM cycling.calendar WHERE race_start_date = '2015-06-13' ALLOW FILTERING;
race_id | race_start_date | race_end_date | race_name ---------+---------------------------------+---------------------------------+---------------- 102 | 2015-06-13 00:00:00.000000+0000 | 2015-06-21 00:00:00.000000+0000 | Tour de Suisse
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You can also pick the columns to display instead of choosing all data.
SELECT category, points, lastname FROM cycling.cyclist_category;
category | points | lastname ---------------+--------+------------ One-day-races | 367 | TIRALONGO One-day-races | 198 | KRUIJSWIJK Time-trial | 182 | TIRALONGO Time-trial | 3 | KRUIJSWIJK Sprint | 39 | KRUIJSWIJK Sprint | 0 | TIRALONGO GC | 1324 | KRUIJSWIJK GC | 1269 | TIRALONGO
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For a large table, limit the number of rows retrieved using
LIMIT
. The default limit is 10,000 rows. To sample data, pick a smaller number. To retrieve more than 10,000 rows, setLIMIT
to a large value.CREATE TABLE cycling.cyclist_name ( id UUID PRIMARY KEY, lastname text, firstname text ); SELECT id, lastname, firstname FROM cycling.cyclist_name LIMIT 3;
id | lastname | firstname --------------------------------------+----------+----------- e7ae5cf3-d358-4d99-b900-85902fda9bb0 | FRAME | Alex fb372533-eb95-4bb4-8685-6ef61e994caa | MATTHEWS | null 5b6962dd-3f90-4c93-8f61-eabfa4a803e2 | VOS | null
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You can fine-tune the display order using the
ORDER BY
clause. The partition key must be defined in theWHERE
clause and theORDER BY
clause defines the clustering column to use for ordering.CREATE TABLE cycling.cyclist_cat_pts ( category text, points int, id UUID,lastname text, PRIMARY KEY (category, points) ); SELECT * FROM cycling.cyclist_cat_pts WHERE category = 'GC' ORDER BY points ASC;
category | points | id | lastname ----------+--------+--------------------------------------+------------ GC | 780 | 829aa84a-4bba-411f-a4fb-38167a987cda | SUTHERLAND GC | 1269 | 220844bf-4860-49d6-9a4b-6b5d3a79cbfb | TIRALONGO
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Tuples are retrieved in their entirety. This example uses
AS
to change the header of the tuple name.SELECT race_name, point_id, lat_long AS CITY_LATITUDE_LONGITUDE FROM cycling.route;
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The
PER PARTITION LIMIT
option sets the maximum number of rows that the query returns from each partition. This is interesting because it allows a query to select a "Top 3" selection if the partitions are separated correctly. Create a table that will sort data into more than one partition and insert some data:CREATE TABLE cycling.rank_by_year_and_name ( race_year int, race_name text, cyclist_name text, rank int, PRIMARY KEY ((race_year, race_name), rank) );
race_year | race_name | rank | cyclist_name -----------+--------------------------------------------+------+---------------------- 2014 | 4th Tour of Beijing | 1 | Phillippe GILBERT 2014 | 4th Tour of Beijing | 2 | Daniel MARTIN 2014 | 4th Tour of Beijing | 3 | Johan Esteban CHAVES 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 1 | Daniel MARTIN 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 2 | Johan Esteban CHAVES 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 3 | Benjamin PRADES 2015 | Giro d'Italia - Stage 11 - Forli > Imola | 1 | Ilnur ZAKARIN 2015 | Giro d'Italia - Stage 11 - Forli > Imola | 2 | Carlos BETANCUR 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 1 | Benjamin PRADES 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 2 | Adam PHELAN 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 3 | Thomas LEBAS
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Now use a
PER PARTITION LIMIT
to get the top two races for each race year and race name pair:SELECT * FROM cycling.rank_by_year_and_name PER PARTITION LIMIT 2;
race_year | race_name | rank | cyclist_name -----------+--------------------------------------------+------+---------------------- 2014 | 4th Tour of Beijing | 1 | Phillippe GILBERT 2014 | 4th Tour of Beijing | 2 | Daniel MARTIN 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 1 | Daniel MARTIN 2014 | Tour of Japan - Stage 4 - Minami > Shinshu | 2 | Johan Esteban CHAVES 2015 | Giro d'Italia - Stage 11 - Forli > Imola | 1 | Ilnur ZAKARIN 2015 | Giro d'Italia - Stage 11 - Forli > Imola | 2 | Carlos BETANCUR 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 1 | Benjamin PRADES 2015 | Tour of Japan - Stage 4 - Minami > Shinshu | 2 | Adam PHELAN