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= Understanding Schemas

== Overview

Ignite has a number of default schemas and supports creating custom schemas.

There are two schemas that are available by default:

- The SYS schema, which contains a number of system views with information about cluster nodes. You can't create tables in this schema. Refer to the link:monitoring-metrics/system-views[System Views] page for further information.
- The <<PUBLIC Schema,PUBLIC schema>>, which is used by default whenever a schema is not specified.

Custom schemas are created in the following cases:

- You can specify custom schemas in the cluster configuration. See <<Custom Schemas>>.
- Ignite creates a schema for each cache created via one of the programming interfaces or XML configuration. See <<Cache and Schema Names>>.


== PUBLIC Schema

The PUBLIC schema is used by default whenever a schema is required and is not specified. For example, when you connect to the cluster via JDBC without setting the schema explicitly, you will connect to the PUBLIC schema.


== Custom Schemas
Custom schemas can be set via the `sqlSchemas` property of `IgniteConfiguration`. You can specify a list of schemas in the configuration before starting your cluster and then create objects in these schemas at runtime.

Below is a configuration example with two custom schemas.


[tabs]
--
tab:XML[]
[source,xml]
----
include::code-snippets/xml/schemas.xml[tags=ignite-config;!discovery, indent=0]
----
tab:Java[]
[source,java]
----
include::{javaCodeDir}/Schemas.java[tags=custom-schemas, indent=0]
----
tab:C#/.NET[]
[source,csharp]
----
include::code-snippets/dotnet/UnderstandingSchemas.cs[tag=schemas,indent=0]
----

tab:C++[unsupported]
--

To connect to a specific schema via, for example, a JDBC driver, provide the schema name in the connection string:

[source,text]
----
jdbc:ignite:thin://127.0.0.1/MY_SCHEMA
----

== Cache and Schema Names
When you create a cache with link:SQL/sql-api#configuring-queryable-fields[queryable fields], you can manipulate the cached data using the link:SQL/sql-api[SQL API]. In SQL terms, each such cache corresponds to a separate schema whose name equals the name of the cache.

Similarly, when you create a table via a DDL statement, you can access it as a key-value cache via Ignite's supported programming interfaces. The name of the corresponding cache can be specified by providing the `CACHE_NAME` parameter in the `WITH` part of the `CREATE TABLE` statement.

[source,sql]
----
CREATE TABLE City (
  ID INT(11),
  Name CHAR(35),
  CountryCode CHAR(3),
  District CHAR(20),
  Population INT(11),
  PRIMARY KEY (ID, CountryCode)
) WITH "backups=1, CACHE_NAME=City";
----

See the link:sql-reference/ddl#create-table[CREATE TABLE] page for more details.

If you do not use this parameter, the cache name is defined in the following format (in capital letters):

....
SQL_<SCHEMA_NAME>_<TABLE_NAME>
....