Retrieval and sorting results 

Using the SELECT command for simple queries.

Querying tables to select data is the reason data is stored in databases. 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 calculate new values based on user-defined functions.

Setting up the example table 

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) );
Insert the data:
INSERT INTO cycling.rank_by_year_and_name (race_year, race_name, cyclist_name, rank) 
   VALUES (2015, 'Tour of Japan - Stage 4 - Minami > Shinshu', 'Benjamin PRADES', 1);
INSERT INTO cycling.rank_by_year_and_name (race_year, race_name, cyclist_name, rank) 
   VALUES (2015, 'Tour of Japan - Stage 4 - Minami > Shinshu', 'Adam PHELAN', 2);
INSERT INTO cycling.rank_by_year_and_name (race_year, race_name, cyclist_name, rank) 
   VALUES (2015, 'Tour of Japan - Stage 4 - Minami > Shinshu', 'Thomas LEBAS', 3);
INSERT INTO cycling.rank_by_year_and_name (race_year, race_name, cyclist_name, rank) 
   VALUES (2015, 'Giro d''Italia - Stage 11 - Forli > Imola', 'Ilnur ZAKARIN', 1);
INSERT INTO cycling.rank_by_year_and_name (race_year, race_name, cyclist_name, rank) 
   VALUES (2015, 'Giro d''Italia - Stage 11 - Forli > Imola', 'Carlos BETANCUR', 2);
INSERT INTO cycling.rank_by_year_and_name (race_year, race_name, cyclist_name, rank) 
   VALUES (2014, '4th Tour of Beijing', 'Phillippe GILBERT', 1);
INSERT INTO cycling.rank_by_year_and_name (race_year, race_name, cyclist_name, rank)  
   VALUES (2014, '4th Tour of Beijing', 'Daniel MARTIN', 2);
INSERT INTO cycling.rank_by_year_and_name (race_year, race_name, cyclist_name, rank)  
   VALUES (2014, '4th Tour of Beijing', 'Johan Esteban CHAVES', 3);

Procedure

  • Use a simple SELECT query to display all data from a table.
    cqlsh> SELECT * FROM cycling.cyclist_category;
    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
          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
  • The example below illustrates how to create a query that uses category as a filter.
    cqlsh> SELECT * FROM cycling.cyclist_category WHERE category = 'SPRINT';


    Note that Cassandra will reject this query if category is not a partition key or clustering column. Queries require a sequential retrieval across the entire cyclist_category table. In a distributed database like Cassandra, 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.

    A query based on lastname can result in satisfactory results if the lastname column is indexed.

  • You can also pick the columns to display instead of choosing all data.
    cqlsh> SELECT category, points, lastname FROM cycling.cyclist_category;
  • 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 set LIMIT to a large value.
    cqlsh> SELECT * From cycling.cyclist_name LIMIT 3;
  • You can fine-tune the display order using the ORDER BY clause. The partition key must be defined in the WHERE clause and the ORDER BY clause defines the clustering column to use for ordering.
    cqlsh> 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;
  • Tuples are retrieved in their entirety. This example uses AS to change the header of the tuple name.
    cqlsh> SELECT race_name, point_id, lat_long AS CITY_LATITUDE_LONGITUDE FROM cycling.route;