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375 lines
19 KiB
Markdown
375 lines
19 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` (or: `information_schema`) is a system database which provides a (somewhat) standardized, [DBMS-agnostic view](https://en.wikipedia.org/wiki/Information_schema) on metadata of database objects. The views in `INFORMATION_SCHEMA` are generally inferior to normal system tables but tools can use them to obtain basic information in a cross-DBMS manner. The structure and content of views in `INFORMATION_SCHEMA` is supposed to evolves in a backwards-compatible way, i.e. only new functionality is added but existing functionality is not changed or removed. In terms of internal implementation, views in `INFORMATION_SCHEMA` usually map to to normal system tables like [system.columns](../../operations/system-tables/columns.md), [system.databases](../../operations/system-tables/databases.md) and [system.tables](../../operations/system-tables/tables.md).
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``` sql
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SHOW TABLES FROM INFORMATION_SCHEMA;
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-- or:
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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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│ KEY_COLUMN_USAGE │
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│ REFERENTIAL_CONSTRAINTS │
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│ SCHEMATA │
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│ TABLES │
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│ VIEWS │
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│ columns │
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│ key_column_usage │
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│ referential_constraints │
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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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- [KEY_COLUMN_USAGE](#key_column_usage)
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- [REFERENTIAL_CONSTRAINTS](#referential_constraints)
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Case-insensitive equivalent views, e.g. `INFORMATION_SCHEMA.columns` are provided for reasons of compatibility with other databases. The same applies to all the columns in these views - both lowercase (for example, `table_name`) and uppercase (`TABLE_NAME`) variants are provided.
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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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- `extra` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `STORED GENERATED` for `MATERIALIZED`-type columns, `VIRTUAL GENERATED` for `ALIAS`-type columns, `DEFAULT_GENERATED` for `DEFAULT`-type columns, or `NULL`.
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**Example**
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Query:
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``` sql
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SELECT table_catalog,
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table_schema,
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table_name,
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column_name,
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ordinal_position,
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column_default,
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is_nullable,
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data_type,
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character_maximum_length,
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character_octet_length,
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numeric_precision,
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numeric_precision_radix,
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numeric_scale,
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datetime_precision,
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character_set_catalog,
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character_set_schema,
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character_set_name,
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collation_catalog,
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collation_schema,
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collation_name,
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domain_catalog,
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domain_schema,
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domain_name,
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column_comment,
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column_type
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FROM INFORMATION_SCHEMA.COLUMNS
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WHERE (table_schema = currentDatabase() OR table_schema = '')
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AND table_name NOT LIKE '%inner%'
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LIMIT 1
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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 catalog_name,
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schema_name,
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schema_owner,
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default_character_set_catalog,
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default_character_set_schema,
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default_character_set_name,
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sql_path
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FROM information_schema.schemata
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WHERE schema_name ilike 'information_schema'
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LIMIT 1
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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` ([String](../../sql-reference/data-types/string.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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- `table_rows` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — The total
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number of rows. NULL if it could not be determined.
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- `data_length` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt64](../../sql-reference/data-types/int-uint.md))) — The size of
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the data on-disk. NULL if it could not be determined.
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- `table_collation` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The table default collation. Always `utf8mb4_0900_ai_ci`.
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- `table_comment` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The comment used when creating the table.
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**Example**
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Query:
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``` sql
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SELECT table_catalog,
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table_schema,
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table_name,
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table_type,
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table_collation,
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table_comment
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FROM INFORMATION_SCHEMA.TABLES
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WHERE (table_schema = currentDatabase() OR table_schema = '')
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AND table_name NOT LIKE '%inner%'
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LIMIT 1
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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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table_collation: utf8mb4_0900_ai_ci
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table_comment:
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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 table_catalog,
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table_schema,
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table_name,
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view_definition,
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check_option,
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is_updatable,
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is_insertable_into,
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is_trigger_updatable,
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is_trigger_deletable,
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is_trigger_insertable_into
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FROM information_schema.views
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WHERE table_schema = currentDatabase()
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LIMIT 1
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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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## KEY_COLUMN_USAGE {#key_column_usage}
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Contains columns from the [system.tables](../../operations/system-tables/tables.md) system table which are restricted by constraints.
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Columns:
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- `constraint_catalog` ([String](../../sql-reference/data-types/string.md)) — Currently unused. Always `def`.
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- `constraint_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the schema (database) to which the constraint belongs.
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- `constraint_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The name of the constraint.
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- `table_catalog` ([String](../../sql-reference/data-types/string.md)) — Currently unused. Always `def`.
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- `table_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the schema (database) to which the table belongs.
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- `table_name` ([String](../../sql-reference/data-types/string.md)) — The name of the table that has the constraint.
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- `column_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The name of the column that has the constraint.
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- `ordinal_position` ([UInt32](../../sql-reference/data-types/int-uint.md)) — Currently unused. Always `1`.
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- `position_in_unique_constraint` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt32](../../sql-reference/data-types/int-uint.md))) — Currently unused. Always `NULL`.
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- `referenced_table_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — Currently unused. Always NULL.
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- `referenced_table_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — Currently unused. Always NULL.
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- `referenced_column_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — Currently unused. Always NULL.
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**Example**
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```sql
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CREATE TABLE test (i UInt32, s String) ENGINE MergeTree ORDER BY i;
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SELECT constraint_catalog,
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constraint_schema,
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constraint_name,
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table_catalog,
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table_schema,
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table_name,
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column_name,
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ordinal_position,
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position_in_unique_constraint,
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referenced_table_schema,
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referenced_table_name,
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referenced_column_name
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FROM information_schema.key_column_usage
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WHERE table_name = 'test'
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FORMAT Vertical;
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```
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Result:
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```
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Row 1:
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──────
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constraint_catalog: def
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constraint_schema: default
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constraint_name: PRIMARY
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table_catalog: def
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table_schema: default
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table_name: test
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column_name: i
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ordinal_position: 1
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position_in_unique_constraint: ᴺᵁᴸᴸ
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referenced_table_schema: ᴺᵁᴸᴸ
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referenced_table_name: ᴺᵁᴸᴸ
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referenced_column_name: ᴺᵁᴸᴸ
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```
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## REFERENTIAL_CONSTRAINTS {#referential_constraints}
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Contains information about foreign keys. Currently returns an empty result (no rows) which is just enough to provide compatibility with 3rd party tools like Tableau Online.
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Columns:
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- `constraint_catalog` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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- `constraint_schema` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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- `constraint_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — Currently unused.
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- `unique_constraint_catalog` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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- `unique_constraint_schema` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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- `unique_constraint_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — Currently unused.
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- `match_option` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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- `update_rule` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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- `delete_rule` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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- `table_name` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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- `referenced_table_name` ([String](../../sql-reference/data-types/string.md)) — Currently unused.
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