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Collection data types (MAP, LIST, and SET)

Synopsis

Use collection data types to specify columns for data objects that can contain more than one value.

LIST

LIST is an ordered collection of elements. All elements in a LIST must be of the same primitive type. Elements can be prepend or append by + operator to a list, removed by - operator, and referenced by their indexes of that list by [] operator.

MAP

MAP is an sorted collection of pairs of elements, a key and a value. The sorting order is based on the key values and is implementation-dependent. With their key values, elements in a MAP can be set by the [] operator, added by the + operator, and removed by the - operator. When queries, the element pairs of a map will be returned in the sorting order.

SET

SET is a sorted collection of elements. The sorting order is implementation-dependent. Elements can be added by + operator and removed by - operator. When queried, the elements of a set will be returned in the sorting order.

Syntax

type_specification ::= { LIST<type> | MAP<key_type:type> | SET<key_type> }

list_literal ::= '[' [ expression ...] ']'

map_literal ::= '{' [ { expression ':' expression } ...] '}'

set_literal ::= '{' [ expression ...] '}'

Where

  • Columns of type LIST, MAP, or SET cannot be part of the PRIMARY KEY.
  • type must be a non-parametric data type or a frozen data type.
  • key_type must be any data type that is allowed in a primary key (Currently FROZEN and all non-parametric data types except BOOL).
  • For map_literal the left-side expression represents the key and the right-side one represents the value.
  • expression is any well formed YCQL expression. See Expression for more information on syntax rules.

Semantics

  • Type parameters must be simple types or frozen types (collections and user-defined types must be frozen to be used as collection parameters).
  • Columns of type LIST, MAP, and SET cannot be part of the PRIMARY KEY.
  • Implicitly, values of collection data types are neither convertible nor comparable to other data types.
  • Each expression in a collection literal must evaluate to a value convertible to the corresponding parameter data type.
  • Comparisons on collection values are not allowed (e.g. in WHERE or IF clauses).
  • Empty collections are treated as null values.

Note

Collections are designed for storing small sets of values that are not expected to grow to arbitrary size (such as phone numbers or addresses for a user rather than posts or messages). While collections of larger sizes are allowed, they may have a significant impact on performance for queries involving them. In particular, some list operations (insert at an index and remove elements) require a read-before-write.

Examples

CREATE TABLE with collections

  • Collection types are used like simple types (except they are not allowed in primary key).
ycqlsh:example> CREATE TABLE users(username TEXT PRIMARY KEY, 
                                  emails SET<TEXT>,
                                  phones MAP<TEXT,TEXT>,
                                  top_cities LIST<TEXT>);

INSERT collection data

  • Collection values are inserted by setting all their elements at once.
ycqlsh:example> INSERT INTO users(username, emails, phones, top_cities) 
               VALUES ('foo', 
                       {'c@example.com', 'a@example.com'}, 
                       {'home' : '999-9999', 'mobile' : '000-0000'}, 
                       ['New York', 'Paris']);

Empty collections are the same as nulls.

ycqlsh:example> INSERT INTO users(username, emails, phones, top_cities) VALUES ('bar', { }, { }, [ ]);

ycqlsh:example> SELECT * FROM users;

 username | emails                             | phones                                     | top_cities
----------+------------------------------------+--------------------------------------------+-----------------------
      bar |                               null |                                       null |                  null
      foo | {'a@example.com', 'c@example.com'} | {'home': '999-9999', 'mobile': '000-0000'} | ['New York', 'Paris']

UPDATE collection column

  • Collection values can be updated by setting all their elements at once.
ycqlsh:example> UPDATE users SET emails = {'bar@example.com'} WHERE username = 'bar';

ycqlsh:example> UPDATE users SET phones = {'home' : '123-45678'} WHERE username = 'bar';

ycqlsh:example> UPDATE users SET top_cities = ['London', 'Tokyo'] WHERE username = 'bar';

ycqlsh:example> SELECT * FROM users;

 username | emails                             | phones                                     | top_cities
----------+------------------------------------+--------------------------------------------+-----------------------
      bar |                {'bar@example.com'} |                      {'home': '123-45678'} |   ['London', 'Tokyo']
      foo | {'a@example.com', 'c@example.com'} | {'home': '999-9999', 'mobile': '000-0000'} | ['New York', 'Paris']

Collection expressions

  • Collection elements can be added with + or removed with -.
ycqlsh:example> UPDATE users SET emails = emails + {'foo@example.com'} WHERE username = 'foo';

ycqlsh:example> UPDATE users SET emails = emails - {'a@example.com', 'c.example.com'} WHERE username = 'foo';

ycqlsh:example> UPDATE users SET phones = phones + {'office' : '333-3333'} WHERE username = 'foo';

ycqlsh:example> SELECT * FROM users;

 username | emails                               | phones                                                           | top_cities
----------+--------------------------------------+------------------------------------------------------------------+-----------------------
      bar |                  {'bar@example.com'} |                                            {'home': '123-45678'} |   ['London', 'Tokyo']
      foo | {'c@example.com', 'foo@example.com'} | {'home': '999-9999', 'mobile': '000-0000', 'office': '333-3333'} | ['New York', 'Paris']
  • To remove map elements only the relevant keys need to be given (as a set).
ycqlsh:example> UPDATE users SET phones = phones - {'home'} WHERE username = 'foo';

ycqlsh:example> SELECT * FROM users;

 username | emails                               | phones                                       | top_cities
----------+--------------------------------------+----------------------------------------------+-----------------------
      bar |                  {'bar@example.com'} |                        {'home': '123-45678'} |   ['London', 'Tokyo']
      foo | {'c@example.com', 'foo@example.com'} | {'mobile': '000-0000', 'office': '333-3333'} | ['New York', 'Paris']
  • List elements can be either prepended or appended.
ycqlsh:example> UPDATE users SET top_cities = top_cities + ['Delhi'] WHERE username = 'foo';

ycqlsh:example> UPDATE users SET top_cities = ['Sunnyvale'] + top_cities WHERE username = 'foo';

ycqlsh:example> UPDATE users SET top_cities = top_cities - ['Paris', 'New York'] WHERE username = 'foo';

ycqlsh:example> SELECT * FROM users;

 username | emails              | phones                                       | top_cities
----------+---------------------+----------------------------------------------+------------------------
      bar | {'bar@example.com'} |                        {'home': '123-45678'} |    ['London', 'Tokyo']
      foo | {'foo@example.com'} | {'mobile': '000-0000', 'office': '333-3333'} | ['Sunnyvale', 'Delhi']

UPDATE map and list elements

  • Maps allow referencing elements by key.
ycqlsh:example> UPDATE users SET phones['mobile'] = '111-1111' WHERE username = 'foo';

ycqlsh:example> UPDATE users SET phones['mobile'] = '345-6789' WHERE username = 'bar' IF phones['mobile'] = null;

ycqlsh:example> SELECT * FROM users;

 username | emails                               | phones                                       | top_cities
----------+--------------------------------------+----------------------------------------------+-----------------------
      bar |                  {'bar@example.com'} |  {'home': '123-45678', 'mobile': '345-6789'} |   ['London', 'Tokyo']
      foo | {'c@example.com', 'foo@example.com'} | {'mobile': '111-1111', 'office': '333-3333'} | ['New York', 'Paris']
  • Lists allow referencing elements by index (numbering starts from 0).
ycqlsh:example> UPDATE users SET top_cities[0] = 'San Francisco' WHERE username = 'bar';

ycqlsh:example> UPDATE users SET top_cities[1] = 'Mumbai' WHERE username = 'bar' IF top_cities[1] = 'Tokyo';

ycqlsh:example> SELECT * FROM users;

 username | emails                               | phones                                       | top_cities
----------+--------------------------------------+----------------------------------------------+-----------------------------
      bar |                  {'bar@example.com'} |  {'home': '123-45678', 'mobile': '345-6789'} | ['San Francisco', 'Mumbai']
      foo | {'c@example.com', 'foo@example.com'} | {'mobile': '111-1111', 'office': '333-3333'} |       ['New York', 'Paris']

See also

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