Apache Cassandra – Part 04 (CQL)

From previous posts we talked about the basics ,installation and cqlsh of Apache Cassandra,

  1. Part 01 – Basics/ Introduction
  2. Part 02 – Installation
  3. Part 03 – cqlsh

From this post I’m going to talk about Cassandra query language, CQL. It’s similar to the SQL what we already know. So let’s have a close look at a comparison of terms before going in to the queries.

Mysql Vs. Cassandra

Data Base – Key Space

Table – Column Family

So next let’s have a close look at the CQL queries one by one,

  • Create Key space : Create a new key space

CREATE KEYSPACE <name> WITH replication = {‘class’: ‘<type>’, ‘replication_factor’: <no-of-replications>} AND durable_writes = ‘<true/false>’

replication – Mandatory : class – type of replication

                                                  replication_factor – no of replicas

durable_writes – Optionaltrue – will write a commit log : default

false – will not write the commit log

1

  • We can check whether the key space has created or not using DESC. DESC KEYSPACES will list all the existing key spaces.

23

  • USE – Same as MySQL, USE will select a Key Space to work on.

4

  • Alter Key space : To change the existing key space

ALTER KEYSPACE<name> WITH replication = {‘class’: ‘<type>’, ‘replication_factor’: <no-of-replications>} AND durable_writes = ‘<true/false>’

5

  • Drop key space : Drop an existing key space

DROP KEYSPACE <name>

6

  • Create column family : Create a new column family/ table

CREATE COLUMNFAMILY<name>(

<column-one> <data-type>,

<column-two> <data-type>,

….

)

8

9

CREATE TABLE<name>(

<column-one> <data-type>,

<column-two> <data-type>,

….

)

11

12

  • Alter table : Change existing table

ALTER TABLE <name> 

ADD  <column-one> <data-type>

13

14

ALTER TABLE <name> 

DROP <column-one>

16

17

  • Drop table : To drop the table from the key space

DROP TABLE <name>

18

19

  • Insert data to the table

INSERT INTO <name>(<column-one>,<column-two>,..) VALUES (‘<value-one>’,'<value-two>’,…)

20

  • Select data from a table

SELECT * FROM <name>

SELECT <column-one>,<column-two>,.. FROM <name>

21

  • Update table : update data in a table

UPDATE <name> SET <column-name-one>=<value-one>, <column-name-two>=<value-two> WHERE <column-name>=<value>

34

  • Delete data : Remove columns or data from table

DELETE <column> FROM <table-name> WHERE <column>=<value>

35

DELETE FROM <table-name> WHERE <column>=<value>

36

  • Truncate table : Delete data from a table

TRUNCATE TABLE <name>

23

24

  • Create index : Indexes can be helpful in increasing the performance. We can create indexes from a non primary key column as below,

CREATE INDEX <name> ON <table-name>(<column-name>)

26

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  • Drop index : We can drop an index as well.

DROP INDEX <name>

28

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  • BATCH : We can execute a set of commands at once as well.

BEGIN BATCH

<query-one>;

<query-two>;

….

APPLY BATCH;

31

32

As you can see Cassandra Query Language is similar to SQL. That’s all for this post. Hope now you have a clear idea about CQL. See you soon with another interesting topic. Thank You!

 

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