Cassandra 集合用于处理任务。您可以在集合中存储多个元素。Cassandra 支持三种类型的集合：
- Set
- List
- Map
Set 集合
Set 集合存储查询时返回排序元素的元素组。
语法：
Create table table_name
(
id int,
Name text,
Email set<text>,
Primary key(id)
);
示例：
下面举个例子来展示Set集合。创建一个具有三列(id, name 和 email)的表“employee”。
use yiibai_ks;
create table employee(
id int,
name text,
email set<text>,
primary key(id)
);
执行上面语句创建表以后如下：
cqlsh:yiibai_ks> create table employee(
... id int,
... name text,
... email set<text>,
... primary key(id)
... );
cqlsh:yiibai_ks> describe employee;
CREATE TABLE yiibai_ks.employee (
id int PRIMARY KEY,
email set<text>,
name text
) WITH bloom_filter_fp_chance = 0.01
AND caching = {'keys': 'ALL', 'rows_per_partition': 'NONE'}
AND comment = ''
AND compaction = {'class': 'org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy', 'max_threshold': '32', 'min_threshold': '4'}
AND compression = {'chunk_length_in_kb': '64', 'class': 'org.apache.cassandra.io.compress.LZ4Compressor'}
AND crc_check_chance = 1.0
AND dclocal_read_repair_chance = 0.1
AND default_time_to_live = 0
AND gc_grace_seconds = 864000
AND max_index_interval = 2048
AND memtable_flush_period_in_ms = 0
AND min_index_interval = 128
AND read_repair_chance = 0.0
AND speculative_retry = '99PERCENTILE';
cqlsh:yiibai_ks>
现在，向上面创建的employee表中插入一些值：
INSERT INTO employee (id, email, name)
VALUES(1, {'yestouu@gmail.com'}, 'yestouu');
INSERT INTO employee (id, email, name)
VALUES(2,{'kanchan@qq.com'}, 'Kanchan');
INSERT INTO employee (id, email, name)
VALUES(3, {'maxsu@126.com'}, 'Maxsu');
执行上面语句结果如下 -
cqlsh:yiibai_ks> INSERT INTO employee (id, email, name)
... VALUES(1, {'yestouu@gmail.com'}, 'yestouu');
cqlsh:yiibai_ks> INSERT INTO employee (id, email, name)
... VALUES(2,{'kanchan@qq.com'}, 'Kanchan');
cqlsh:yiibai_ks> INSERT INTO employee (id, email, name)
... VALUES(3, {'maxsu@126.com'}, 'Maxsu');
cqlsh:yiibai_ks> select * from employee;
id | email | name
----+-----------------------+---------
1 | {'yestouu@gmail.com'} | yestouu
2 | {'kanchan@qq.com'} | Kanchan
3 | {'maxsu@126.com'} | Maxsu
(3 rows)
cqlsh:yiibai_ks>
List 集合
当元素的顺序重要时，使用列表(List)集合。我们以扩展上面示例中的employee表并新增一列department为例。
alter table employee add department list<text>;
执行结果如下 -
cqlsh:yiibai_ks> alter table employee add department list<text>;
cqlsh:yiibai_ks> select * from employee;
id | department | email | name
----+------------+-----------------------+---------
1 | null | {'yestouu@gmail.com'} | yestouu
2 | null | {'kanchan@qq.com'} | Kanchan
3 | null | {'maxsu@126.com'} | Maxsu
(3 rows)
cqlsh:yiibai_ks>
现在添加新列(department)。在新列“department”中插入一些值。
INSERT INTO employee (id, email, name, department) VALUES(4, {'sweetsu@gmail.com'}, 'Sweetsu', ['IT Devopment']);
执行上面语句结果如下 -
cqlsh:yiibai_ks> alter table employee add department list<text>;
cqlsh:yiibai_ks> select * from employee;
id | department | email | name
----+------------+-----------------------+---------
1 | null | {'yestouu@gmail.com'} | yestouu
2 | null | {'kanchan@qq.com'} | Kanchan
3 | null | {'maxsu@126.com'} | Maxsu
(3 rows)
cqlsh:yiibai_ks> INSERT INTO employee (id, email, name, department) VALUES(4, {'sweetsu@gmail.com'}, 'Sweetsu', ['IT Devopment']);
cqlsh:yiibai_ks> select * from employee;
id | department | email | name
----+------------------+-----------------------+---------
1 | null | {'yestouu@gmail.com'} | yestouu
2 | null | {'kanchan@qq.com'} | Kanchan
4 | ['IT Devopment'] | {'sweetsu@gmail.com'} | Sweetsu
3 | null | {'maxsu@126.com'} | Maxsu
(4 rows)
cqlsh:yiibai_ks>
Map 集合
Map 集合用于存储键值对。它将一件事映射到另一件事。例如，如果要将必备课程名称来保存课程信息，则可以使用Map集合。
例子：
创建一个名为“course”的表。
create table course(
id int,
prereq map<text, text>,
primary key(id)
);
-- 插入数据
INSERT into course(id,prereq) VALUES(1, {'Programming':'CPP&&Java', 'Network':'Artificail Intelligence'});
现在创建表，在Map集合类型中插入一些数据。
输出：
cqlsh:yiibai_ks> create table course(
... id int,
... prereq map<text, text>,
... primary key(id)
... );
cqlsh:yiibai_ks> INSERT into course(id,prereq) VALUES(1, {'Programming':'CPP&&Java', 'Network':'Artificail Intelligence'});
cqlsh:yiibai_ks> select * from course;
id | prereq
----+--------------------------------------------------------------------
1 | {'Network': 'Artificail Intelligence', 'Programming': 'CPP&&Java'}
(1 rows)
cqlsh:yiibai_ks>