This document describes the basic concepts of the Continuous Query Language (CQL) of Odysseus and shows how to use the language. The Continuous Query Language (CQL) is a SQL based declarative query language. This document shows how to formulate queries with CQL.

Create Streams

The create stream statement is used to tell Odysseus where the data comes from, this normally opens a connection to a source, e.g. a sensor or server.

The stream always consists of a name (here: "category") and a schema:

CREATE STREAM category (id INTEGER, name STRING, description STRING, parentid INTEGER) ....

...

Odysseus has a built-in byte-based format for transfering data. This is, for example, used by the nexmark example. This is called a "CHANNEL"-connection and looks like follows:

CREATE STREAM nexmark:person (timestamp STARTTIMESTAMP,id INTEGER,name STRING,email STRING,creditcard STRING,city STRING,state STRING) CHANNEL localhost : 65440

...

However, the recommended and new way is a generic access, which offers different protocols, wrappers etc. as described in Access framework. An example would be: 

CREATE STREAM nexmark:person (timestamp STARTTIMESTAMP, id INTEGER, name STRING, email STRING, creditcard STRING, city STRING, state STRING)
    WRAPPER 'GenericPush' 
    PROTOCOL 'SizeByteBuffer'
    TRANSPORT 'NonBlockingTcp'
    DATAHANDLER 'Tuple'
    OPTIONS ( 'port' '65440', 'host' 'odysseus.offis.uni-oldenburg.de', 'ByteOrder' 'Little_Endian')

...

CREATE SINK writeout (timestamp STARTTIMESTAMP, auction INTEGER, bidder INTEGER, datetime LONG,    price DOUBLE)
    WRAPPER 'GenericPush'
    PROTOCOL 'CSV'
    TRANSPORT 'File'
    DATAHANDLER 'Tuple'
    OPTIONS ( 'filename' 'E:\test')

...

Drop Streams

You can drop a stream with:

DROP STREAM category

Since this statement would return an error if the stream "category" does not exist, you can add "IF EXISTS" to avoid this error (it checks, if the stream is existing before running the drop)

DROP STREAM category IF EXISTS

...

You can drop a sink with:

DROP SINK category

Since this statement would return an error if the stream "category" does not exist, you can add "IF EXISTS" to avoid this error (it checks, if the sink is existing before running the drop)

DROP SINK category IF EXISTS

...

We use the following example to explain basic details of CQL-Query.

ATTENTION: Currently, the * notation is not allowed for aggregation functions. So instead of count(*) use count(attribute). The parser error is not very helpful in this case: "Caused by: de.uniol.inf.is.odysseus.parser.cql.parser.ParseException: Encountered " "SELECT" "SELECT "" at line 1, column 1. Was expecting:   "REVOKE" ..."

SELECT auction, AVG(price) AS aprice 
FROM bid [SIZE 60 MINUTES ADVANCE 1 MINUTE TIME]
WHERE auction > 10 
GROUP BY auction 
HAVING aprice<100.0

...