ClickHouse/docs/en/operations/system-tables/information_schema.md

---
slug: /en/operations/system-tables/information_schema
---
# INFORMATION_SCHEMA

`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).

``` sql
SHOW TABLES FROM INFORMATION_SCHEMA;

-- or:
SHOW TABLES FROM information_schema;
```

``` text
┌─name────────────────────┐
│ COLUMNS                 │
│ KEY_COLUMN_USAGE        │
│ REFERENTIAL_CONSTRAINTS │
│ SCHEMATA                │
│ TABLES                  │
│ VIEWS                   │
│ columns                 │
│ key_column_usage        │
│ referential_constraints │
│ schemata                │
│ tables                  │
│ views                   │
└─────────────────────────┘
```

`INFORMATION_SCHEMA` contains the following views:

- [COLUMNS](#columns)
- [SCHEMATA](#schemata)
- [TABLES](#tables)
- [VIEWS](#views)
- [KEY_COLUMN_USAGE](#key_column_usage)
- [REFERENTIAL_CONSTRAINTS](#referential_constraints)

Case-insensitive equivalent views, e.g. `INFORMATION_SCHEMA.columns` are provided for reasons of compatibility with other databases.

## COLUMNS {#columns}

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.

Columns:

- `table_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
- `table_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
- `table_name` ([String](../../sql-reference/data-types/string.md)) — Table name.
- `column_name` ([String](../../sql-reference/data-types/string.md)) — Column name.
- `ordinal_position` ([UInt64](../../sql-reference/data-types/int-uint.md)) — Ordinal position of a column in a table starting with 1.
- `column_default` ([String](../../sql-reference/data-types/string.md)) — Expression for the default value, or an empty string if it is not defined.
- `is_nullable` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column type is `Nullable`.
- `data_type` ([String](../../sql-reference/data-types/string.md)) — Column type.
- `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.
- `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.
- `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.
- `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.
- `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.
- `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.
- `character_set_catalog` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `character_set_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `character_set_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `collation_catalog` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `collation_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `collation_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `domain_catalog` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `domain_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `domain_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.

**Example**

Query:

``` sql
SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE (table_schema=currentDatabase() OR table_schema='') AND table_name NOT LIKE '%inner%' LIMIT 1 FORMAT Vertical;
```

Result:

``` text
Row 1:
──────
table_catalog:            default
table_schema:             default
table_name:               describe_example
column_name:              id
ordinal_position:         1
column_default:
is_nullable:              0
data_type:                UInt64
character_maximum_length: ᴺᵁᴸᴸ
character_octet_length:   ᴺᵁᴸᴸ
numeric_precision:        64
numeric_precision_radix:  2
numeric_scale:            0
datetime_precision:       ᴺᵁᴸᴸ
character_set_catalog:    ᴺᵁᴸᴸ
character_set_schema:     ᴺᵁᴸᴸ
character_set_name:       ᴺᵁᴸᴸ
collation_catalog:        ᴺᵁᴸᴸ
collation_schema:         ᴺᵁᴸᴸ
collation_name:           ᴺᵁᴸᴸ
domain_catalog:           ᴺᵁᴸᴸ
domain_schema:            ᴺᵁᴸᴸ
domain_name:              ᴺᵁᴸᴸ
```

## SCHEMATA {#schemata}

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.

Columns:

- `catalog_name` ([String](../../sql-reference/data-types/string.md)) — The name of the database.
- `schema_name` ([String](../../sql-reference/data-types/string.md)) — The name of the database.
- `schema_owner` ([String](../../sql-reference/data-types/string.md)) — Schema owner name, always `'default'`.
- `default_character_set_catalog` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `default_character_set_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `default_character_set_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.
- `sql_path` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — `NULL`, not supported.

**Example**

Query:

``` sql
SELECT * FROM information_schema.schemata WHERE schema_name ILIKE 'information_schema' LIMIT 1 FORMAT Vertical;
```

Result:

``` text
Row 1:
──────
catalog_name:                  INFORMATION_SCHEMA
schema_name:                   INFORMATION_SCHEMA
schema_owner:                  default
default_character_set_catalog: ᴺᵁᴸᴸ
default_character_set_schema:  ᴺᵁᴸᴸ
default_character_set_name:    ᴺᵁᴸᴸ
sql_path:                      ᴺᵁᴸᴸ
```

## TABLES {#tables}

Contains columns read from the [system.tables](../../operations/system-tables/tables.md) system table.

Columns:

- `table_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
- `table_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
- `table_name` ([String](../../sql-reference/data-types/string.md)) — Table name.
- `table_type` ([String](../../sql-reference/data-types/string.md)) — Table type. Possible values:
    - `BASE TABLE`
    - `VIEW`
    - `FOREIGN TABLE`
    - `LOCAL TEMPORARY`
    - `SYSTEM VIEW`
- `table_comment` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The comment used when creating the table.
- `table_collation` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The table default collation. Always `utf8mb4`.

**Example**

Query:

``` sql
SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE (table_schema = currentDatabase() OR table_schema = '') AND table_name NOT LIKE '%inner%' LIMIT 1 FORMAT Vertical;
```

Result:

``` text
Row 1:
──────
table_catalog: default
table_schema:  default
table_name:    describe_example
table_type:    BASE TABLE
```

## VIEWS {#views}

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.

Columns:

- `table_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
- `table_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the database in which the table is located.
- `table_name` ([String](../../sql-reference/data-types/string.md)) — Table name.
- `view_definition` ([String](../../sql-reference/data-types/string.md)) — `SELECT` query for view.
- `check_option` ([String](../../sql-reference/data-types/string.md)) — `NONE`, no checking.
- `is_updatable` ([Enum8](../../sql-reference/data-types/enum.md)) — `NO`, the view is not updated.
- `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:
    - `NO` — The created view is not materialized.
    - `YES` — The created view is materialized.
- `is_trigger_updatable` ([Enum8](../../sql-reference/data-types/enum.md)) — `NO`, the trigger is not updated.
- `is_trigger_deletable` ([Enum8](../../sql-reference/data-types/enum.md)) — `NO`, the trigger is not deleted.
- `is_trigger_insertable_into` ([Enum8](../../sql-reference/data-types/enum.md)) — `NO`, no data is inserted into the trigger.

**Example**

Query:

``` sql
CREATE VIEW v (n Nullable(Int32), f Float64) AS SELECT n, f FROM t;
CREATE MATERIALIZED VIEW mv ENGINE = Null AS SELECT * FROM system.one;
SELECT * FROM information_schema.views WHERE table_schema = currentDatabase() LIMIT 1 FORMAT Vertical;
```

Result:

``` text
Row 1:
──────
table_catalog:              default
table_schema:               default
table_name:                 mv
view_definition:            SELECT * FROM system.one
check_option:               NONE
is_updatable:               NO
is_insertable_into:         YES
is_trigger_updatable:       NO
is_trigger_deletable:       NO
is_trigger_insertable_into: NO
```

## KEY_COLUMN_USAGE (#key_column_usage)

It was added for compatibility with third party tools such as Tableau Online. Contains only the primary keys columns read from [system.columns](../../operations/system-tables/columns.md).

Columns:

- `constraint_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the catalog to which the constraint belongs. This value is always `def`.
- `constraint_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the schema (database) to which the constraint belongs.
- `constraint_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The name of the constraint.
- `table_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the catalog to which the table belongs. This value is always `def`.
- `table_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the schema (database) to which the table belongs.
- `table_name` ([String](../../sql-reference/data-types/string.md)) — The name of the table that has the constraint.
- `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.
- `ordinal_position` ([UInt32](../../sql-reference/data-types/int-uint.md)) — The column's position within the constraint, not the column's position within the table. Column positions are numbered beginning with 1.
- `position_in_unique_constraint` ([Nullable](../../sql-reference/data-types/nullable.md)([UInt32](../../sql-reference/data-types/int-uint.md))) — Always `NULL`.
- `referenced_table_schema` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The name of the schema referenced by the constraint. Always `NULL`.
- `referenced_table_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The name of the table referenced by the constraint. Always `NULL`.
- `referenced_column_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The name of the column referenced by the constraint. Always `NULL`.

**Example**

```sql
CREATE TABLE test (i UInt32, s String) ENGINE MergeTree ORDER BY i;
SELECT * FROM information_schema.key_column_usage WHERE table_name = 'test' FORMAT Vertical;
```

Result:

```
Row 1:
──────
referenced_table_schema: ᴺᵁᴸᴸ
referenced_table_name:   ᴺᵁᴸᴸ
referenced_column_name:  ᴺᵁᴸᴸ
table_schema:            default
table_name:              test
column_name:             i
ordinal_position:        1
constraint_name:         PRIMARY
```

## REFERENTIAL_CONSTRAINTS (#referential_constraints)

It was added for compatibility with third party tools such as Tableau Online. Reads no data by design, selects from this view will always yield an empty result set.

Columns:

- `constraint_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the catalog to which the constraint belongs.
- `constraint_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the schema (database) to which the constraint belongs.
- `constraint_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The name of the constraint.
- `unique_constraint_catalog` ([String](../../sql-reference/data-types/string.md)) — The name of the catalog containing the unique constraint that the constraint references.
- `unique_constraint_schema` ([String](../../sql-reference/data-types/string.md)) — The name of the schema containing the unique constraint that the constraint references.
- `unique_constraint_name` ([Nullable](../../sql-reference/data-types/nullable.md)([String](../../sql-reference/data-types/string.md))) — The name of the unique constraint that the constraint references.
- `match_option` ([String](../../sql-reference/data-types/string.md)) — The value of the constraint `MATCH` attribute.
- `update_rule` ([String](../../sql-reference/data-types/string.md)) — The value of the constraint `ON UPDATE` attribute.
- `delete_rule` ([String](../../sql-reference/data-types/string.md)) — The value of the constraint `ON DELETE` attribute.
- `table_name` ([String](../../sql-reference/data-types/string.md)) — The name of the table.
- `referenced_table_name` ([String](../../sql-reference/data-types/string.md)) — The name of the table referenced by the constraint.