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214 lines
11 KiB
Markdown
214 lines
11 KiB
Markdown
---
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slug: /en/operations/system-tables/information_schema
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---
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# INFORMATION_SCHEMA
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`INFORMATION_SCHEMA` (`information_schema`) is a system database that contains views. Using these views, you can get information about the metadata of database objects. These views read data from the columns of the [system.columns](../../operations/system-tables/columns.md), [system.databases](../../operations/system-tables/databases.md) and [system.tables](../../operations/system-tables/tables.md) system tables.
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The structure and composition of system tables may change in different versions of the product, but the support of the `information_schema` makes it possible to change the structure of system tables without changing the method of access to metadata. Metadata requests do not depend on the DBMS used.
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``` sql
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SHOW TABLES FROM INFORMATION_SCHEMA;
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```
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``` text
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┌─name─────┐
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│ COLUMNS │
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│ SCHEMATA │
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│ TABLES │
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│ VIEWS │
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└──────────┘
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```
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`INFORMATION_SCHEMA` contains the following views:
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- [COLUMNS](#columns)
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- [SCHEMATA](#schemata)
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- [TABLES](#tables)
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- [VIEWS](#views)
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## COLUMNS {#columns}
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Contains columns read from the [system.columns](../../operations/system-tables/columns.md) system table and columns that are not supported in ClickHouse or do not make sense (always `NULL`), but must be by the standard.
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Columns:
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- `table_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
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- `table_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
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- `table_name` ([String](../../sql-reference/data-types/string.md)) — Table name.
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- `column_name` ([String](../../sql-reference/data-types/string.md)) — Column name.
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- `ordinal_position` ([UInt64](../../sql-reference/data-types/int-uint.md)) — Ordinal position of a column in a table starting with 1.
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- `column_default` ([String](../../sql-reference/data-types/string.md)) — Expression for the default value, or an empty string if it is not defined.
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- `is_nullable` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column type is `Nullable`.
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- `data_type` ([String](../../sql-reference/data-types/string.md)) — Column type.
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- `character_maximum_length` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — Maximum length in bytes for binary data, character data, or text data and images. In ClickHouse makes sense only for `FixedString` data type. Otherwise, the `NULL` value is returned.
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- `character_octet_length` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — Maximum length in bytes for binary data, character data, or text data and images. In ClickHouse makes sense only for `FixedString` data type. Otherwise, the `NULL` value is returned.
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- `numeric_precision` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — Accuracy of approximate numeric data, exact numeric data, integer data, or monetary data. In ClickHouse it is bit width for integer types and decimal precision for `Decimal` types. Otherwise, the `NULL` value is returned.
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- `numeric_precision_radix` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — The base of the number system is the accuracy of approximate numeric data, exact numeric data, integer data or monetary data. In ClickHouse it's 2 for integer types and 10 for `Decimal` types. Otherwise, the `NULL` value is returned.
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- `numeric_scale` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — The scale of approximate numeric data, exact numeric data, integer data, or monetary data. In ClickHouse makes sense only for `Decimal` types. Otherwise, the `NULL` value is returned.
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- `datetime_precision` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — Decimal precision of `DateTime64` data type. For other data types, the `NULL` value is returned.
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- `character_set_catalog` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `character_set_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `character_set_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `collation_catalog` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `collation_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `collation_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `domain_catalog` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `domain_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `domain_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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**Example**
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Query:
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``` sql
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SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE (table_schema=currentDatabase() OR table_schema='') AND table_name NOT LIKE '%inner%' LIMIT 1 FORMAT Vertical;
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```
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Result:
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``` text
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Row 1:
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──────
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table_catalog: default
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table_schema: default
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table_name: describe_example
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column_name: id
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ordinal_position: 1
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column_default:
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is_nullable: 0
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data_type: UInt64
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character_maximum_length: ᴺᵁᴸᴸ
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character_octet_length: ᴺᵁᴸᴸ
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numeric_precision: 64
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numeric_precision_radix: 2
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numeric_scale: 0
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datetime_precision: ᴺᵁᴸᴸ
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character_set_catalog: ᴺᵁᴸᴸ
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character_set_schema: ᴺᵁᴸᴸ
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character_set_name: ᴺᵁᴸᴸ
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collation_catalog: ᴺᵁᴸᴸ
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collation_schema: ᴺᵁᴸᴸ
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collation_name: ᴺᵁᴸᴸ
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domain_catalog: ᴺᵁᴸᴸ
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domain_schema: ᴺᵁᴸᴸ
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domain_name: ᴺᵁᴸᴸ
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```
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## SCHEMATA {#schemata}
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Contains columns read from the [system.databases](../../operations/system-tables/databases.md) system table and columns that are not supported in ClickHouse or do not make sense (always `NULL`), but must be by the standard.
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Columns:
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- `catalog_name` ([String](../../sql-reference/data-types/string.md)) — The name of the database.
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- `schema_name` ([String](../../sql-reference/data-types/string.md)) — The name of the database.
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- `schema_owner` ([String](../../sql-reference/data-types/string.md)) — Schema owner name, always `'default'`.
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- `default_character_set_catalog` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `default_character_set_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `default_character_set_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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- `sql_path` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
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**Example**
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Query:
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``` sql
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SELECT * FROM information_schema.schemata WHERE schema_name ILIKE 'information_schema' LIMIT 1 FORMAT Vertical;
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```
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Result:
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``` text
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Row 1:
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──────
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catalog_name: INFORMATION_SCHEMA
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schema_name: INFORMATION_SCHEMA
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schema_owner: default
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default_character_set_catalog: ᴺᵁᴸᴸ
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default_character_set_schema: ᴺᵁᴸᴸ
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default_character_set_name: ᴺᵁᴸᴸ
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sql_path: ᴺᵁᴸᴸ
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```
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## TABLES {#tables}
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Contains columns read from the [system.tables](../../operations/system-tables/tables.md) system table.
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Columns:
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- `table_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
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- `table_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
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- `table_name` ([String](../../sql-reference/data-types/string.md)) — Table name.
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- `table_type` ([Enum8](../../sql-reference/data-types/enum.md)) — Table type. Possible values:
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- `BASE TABLE`
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- `VIEW`
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- `FOREIGN TABLE`
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- `LOCAL TEMPORARY`
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- `SYSTEM VIEW`
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**Example**
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Query:
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``` sql
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SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE (table_schema = currentDatabase() OR table_schema = '') AND table_name NOT LIKE '%inner%' LIMIT 1 FORMAT Vertical;
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```
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Result:
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``` text
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Row 1:
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──────
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table_catalog: default
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table_schema: default
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table_name: describe_example
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table_type: BASE TABLE
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```
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## VIEWS {#views}
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Contains columns read from the [system.tables](../../operations/system-tables/tables.md) system table, when the table engine [View](../../engines/table-engines/special/view.md) is used.
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Columns:
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- `table_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
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- `table_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
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- `table_name` ([String](../../sql-reference/data-types/string.md)) — Table name.
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- `view_definition` ([String](../../sql-reference/data-types/string.md)) — `SELECT` query for view.
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- `check_option` ([String](../../sql-reference/data-types/string.md)) — `NONE`, no checking.
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- `is_updatable` ([Enum8](../../sql-reference/data-types/enum.md)) — `NO`, the view is not updated.
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- `is_insertable_into` ([Enum8](../../sql-reference/data-types/enum.md)) — Shows whether the created view is [materialized](../../sql-reference/statements/create/view.md/#materialized-view). Possible values:
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- `NO` — The created view is not materialized.
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- `YES` — The created view is materialized.
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- `is_trigger_updatable` ([Enum8](../../sql-reference/data-types/enum.md)) — `NO`, the trigger is not updated.
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- `is_trigger_deletable` ([Enum8](../../sql-reference/data-types/enum.md)) — `NO`, the trigger is not deleted.
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- `is_trigger_insertable_into` ([Enum8](../../sql-reference/data-types/enum.md)) — `NO`, no data is inserted into the trigger.
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**Example**
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Query:
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``` sql
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CREATE VIEW v (n Nullable(Int32), f Float64) AS SELECT n, f FROM t;
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CREATE MATERIALIZED VIEW mv ENGINE = Null AS SELECT * FROM system.one;
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SELECT * FROM information_schema.views WHERE table_schema = currentDatabase() LIMIT 1 FORMAT Vertical;
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```
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Result:
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``` text
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Row 1:
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──────
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table_catalog: default
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table_schema: default
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table_name: mv
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view_definition: SELECT * FROM system.one
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check_option: NONE
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is_updatable: NO
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is_insertable_into: YES
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is_trigger_updatable: NO
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is_trigger_deletable: NO
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is_trigger_insertable_into: NO
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```
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