==============
Querying Crate
==============

This section provides an overview on how to query documents using SQL.  See
:ref:`sql_ddl` for information about `Table creation` and other Data Definition
statements.

Retrieving Data
===============

Retrieving data from ``Crate`` is done by using a SQL ``SELECT`` statement.
The response to a ``SELECT`` query contains the column names of the result and
the actual result rows as a two-dimensional array of values.

A simple select::

    cr> select name, position from locations order by "_id" limit 2
    +-------------------+----------+
    | name              | position |
    +-------------------+----------+
    | North West Ripple | 1        |
    | Arkintoofle Minor | 3        |
    +-------------------+----------+

If the '*' operator is used, all properties defined in the schema are returned
for each object::

    cr> select * from locations order by "_id" limit 1 offset 1
    +--------------------------+--------------...---+--------+-------------------+----------+------+
    | date                     | description        | kind   | name              | position | race |
    +--------------------------+--------------...---+--------+-------------------+----------+------+
    | 1979-10-12T00:00:00.000Z | Motivated by ...s. | Planet | Arkintoofle Minor | 3        | None |
    +--------------------------+--------------...---+--------+-------------------+----------+------+

Aliases can be used to change the output name of the columns::

    cr> select name as n from locations order by "_id" limit 1
    +-------------------+
    | n                 |
    +-------------------+
    | North West Ripple |
    +-------------------+

Where Clause
------------

A simple where clause example using an equality operator::

    cr> select description from locations where "_id" = '1'
    +-----------------------------------------...-------------------------------------------+
    | description                                                                           |
    +-----------------------------------------...-------------------------------------------+
    | Relative to life on NowWhat, living on a...er by a factor of about seventeen million. |
    +-----------------------------------------...-------------------------------------------+

Usual comparison operators to be used in filters are available for
string, integer/long, float/double and date type:

    ===========  =================================
    Operator     Description
    ===========  =================================
    <            less than
    >            greater than
    <=           less than or equal to
    >=           greater than or equal to
    =            equal
    <>           not equal
    !=           not equal - same as <>
    is not null  field is not null and not missing
    is null      field is null or missing
    ===========  =================================

.. note::

  The field 'name' used for the request above is defined as
  'not_analyzed' in the schema. For an 'analyzed' field the result may
  differ depending on the used Analyzer/Tokenizer. For details
  regarding analysis please refer to the `elasticsearch analysis`_
  site.

.. _`elasticsearch analysis`: http://www.elasticsearch.org/guide/reference/index-modules/analysis/

For strings a lexicographical comparison is performed based on the
Lucene TermRangeQuery::

    cr> select name from locations where name > 'Argabuthon' order by name
    +------------------------------------+
    | name                               |
    +------------------------------------+
    | Arkintoofle Minor                  |
    | Bartledan                          |
    | Galactic Sector QQ7 Active J Gamma |
    | North West Ripple                  |
    | Outer Eastern Rim                  |
    +------------------------------------+

For details please refer to the `Apache Lucene`_ site.

.. _`Apache Lucene`: http://lucene.apache.org/core/4_0_0/core/org/apache/lucene/search/Query.html


In most SQL databases there is a distinction between `NULL` and empty string.
Since Crate is a schemaless document oriented database there is the third
possibility that a row doesn't contain a field. Due to this `is null` will
return all rows where the fields value is `NULL` or where the field doesn't
exist at all. `is not null` also behaves this way. E.g. the following query
returns 2 rows since the `is null` filter matches all rows and a limit is
specified::

    cr> select name from locations where missing is null order by "_id" limit 2
    +-------------------+
    | name              |
    +-------------------+
    | North West Ripple |
    | Arkintoofle Minor |
    +-------------------+

Number and date field comparison behave as expected from standard SQL.
The following example uses one of the supported ISO date formats::

    cr> select date, position from locations where date <= '1979-10-12' and
    ... position < 3 order by position
    +--------------------------+----------+
    | date                     | position |
    +--------------------------+----------+
    | 1979-10-12T00:00:00.000Z | 1        |
    | 1979-10-12T00:00:00.000Z | 2        |
    +--------------------------+----------+

All supported default ISO date formats are comparable.
For a full list please refer to the `elasticsearch date-format`_ site.

.. _`elasticsearch date-format`: http://www.elasticsearch.org/guide/reference/mapping/date-format/

For custom date types, or defined date formats in the object mapping
the corresponding format should be used for a comparison. Otherwise
the operation may fail.

Inner/Nested Objects
--------------------

``Crate`` supports an ``object`` data type, used for simple storing a whole object into a
column and it's even possible to select and query for properties of such objects.

Select a property of an inner object::

    cr> select name, race['name'] from locations where name = 'Bartledan'
    +-----------+----------------+
    | name      | race['name']   |
    +-----------+----------------+
    | Bartledan | Bartledannians |
    +-----------+----------------+


Query for a property of an inner object::

    cr> select name, race['name'] from locations where race['name'] = 'Bartledannians'
    +-----------+----------------+
    | name      | race['name']   |
    +-----------+----------------+
    | Bartledan | Bartledannians |
    +-----------+----------------+

Data Aggregation
----------------

Currently the only supported aggregation function in `crate` is `count(*)`
without any groupings::

    cr> select count(*) from locations
    +----------+
    | COUNT(*) |
    +----------+
    | 13       |
    +----------+

Inserting data
==============

Inserting data to ``Crate`` is done by using the SQL ``INSERT`` statement.

.. note::

    The column list at ``Crate`` is always ordered alphabetically by column name
    and so must the inserted column values.

Inserting a row::

    cr> insert into locations values ('2013-09-12T21:43:59.000Z', 'Blagulon Kappa is the planet to which the police are native.', 'Planet', 'Blagulon Kappa', 7)

Inserting multiple rows at once (aka. bulk insert) can be done by defining
multiple values for the ``INSERT`` statement::

    cr> insert into locations (date, description, kind, name, position) values
    ... ('2013-09-12T21:43:59.000Z', 'Blagulon Kappa is the planet to which the police are native.', 'Planet', 'Blagulon Kappa', 7),
    ... ('2013-09-13T16:43:59.000Z', 'Brontitall is a planet with a warm, rich atmosphere and no mountains.', 'Planet', 'Brontitall', 10)


Updating data
=============

In order to update documents in `Crate` the SQL `UPDATE` statement can be
used::

    cr> update locations set description = 'Updated description' where name = 'Blagulon Kappa'

Updating nested objects is also supported::

    cr> update locations set race['name'] = 'Human' where name = 'Bartledan'

.. note::

    If the same documents are updated concurrently an VersionConflictException
    might occur. Crate contains a retry logic that tries to resolve the
    conflict automatically. But if it fails more than 3 times the error is
    returned to the user.

Deleting data
=============

Deleting rows in Crate is done using the SQL ``DELETE`` statement::

    cr> delete from locations where position > 3
