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90 lines
5.6 KiB
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90 lines
5.6 KiB
Markdown
# system.columns {#system-columns}
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Contains information about columns in all the tables.
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You can use this table to get information similar to the [DESCRIBE TABLE](../../sql-reference/statements/misc.md#misc-describe-table) query, but for multiple tables at once.
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Columns from [temporary tables](../../sql-reference/statements/create/table.md#temporary-tables) are visible in the `system.columns` only in those session where they have been created. They are shown with the empty `database` field.
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The `system.columns` table contains the following columns (the column type is shown in brackets):
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- `database` ([String](../../sql-reference/data-types/string.md)) — Database name.
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- `table` ([String](../../sql-reference/data-types/string.md)) — Table name.
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- `name` ([String](../../sql-reference/data-types/string.md)) — Column name.
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- `type` ([String](../../sql-reference/data-types/string.md)) — Column type.
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- `position` ([UInt64](../../sql-reference/data-types/int-uint.md)) — Ordinal position of a column in a table starting with 1.
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- `default_kind` ([String](../../sql-reference/data-types/string.md)) — Expression type (`DEFAULT`, `MATERIALIZED`, `ALIAS`) for the default value, or an empty string if it is not defined.
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- `default_expression` ([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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- `data_compressed_bytes` ([UInt64](../../sql-reference/data-types/int-uint.md)) — The size of compressed data, in bytes.
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- `data_uncompressed_bytes` ([UInt64](../../sql-reference/data-types/int-uint.md)) — The size of decompressed data, in bytes.
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- `marks_bytes` ([UInt64](../../sql-reference/data-types/int-uint.md)) — The size of marks, in bytes.
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- `comment` ([String](../../sql-reference/data-types/string.md)) — Comment on the column, or an empty string if it is not defined.
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- `is_in_partition_key` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column is in the partition expression.
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- `is_in_sorting_key` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column is in the sorting key expression.
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- `is_in_primary_key` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column is in the primary key expression.
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- `is_in_sampling_key` ([UInt8](../../sql-reference/data-types/int-uint.md)) — Flag that indicates whether the column is in the sampling key expression.
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- `compression_codec` ([String](../../sql-reference/data-types/string.md)) — Compression codec name.
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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 bitness 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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**Example**
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```sql
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SELECT * FROM system.columns LIMIT 2 FORMAT Vertical;
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```
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```text
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Row 1:
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──────
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database: INFORMATION_SCHEMA
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table: COLUMNS
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name: table_catalog
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type: String
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position: 1
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default_kind:
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default_expression:
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data_compressed_bytes: 0
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data_uncompressed_bytes: 0
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marks_bytes: 0
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comment:
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is_in_partition_key: 0
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is_in_sorting_key: 0
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is_in_primary_key: 0
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is_in_sampling_key: 0
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compression_codec:
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character_octet_length: ᴺᵁᴸᴸ
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numeric_precision: ᴺᵁᴸᴸ
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numeric_precision_radix: ᴺᵁᴸᴸ
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numeric_scale: ᴺᵁᴸᴸ
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datetime_precision: ᴺᵁᴸᴸ
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Row 2:
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──────
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database: INFORMATION_SCHEMA
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table: COLUMNS
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name: table_schema
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type: String
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position: 2
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default_kind:
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default_expression:
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data_compressed_bytes: 0
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data_uncompressed_bytes: 0
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marks_bytes: 0
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comment:
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is_in_partition_key: 0
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is_in_sorting_key: 0
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is_in_primary_key: 0
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is_in_sampling_key: 0
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compression_codec:
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character_octet_length: ᴺᵁᴸᴸ
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numeric_precision: ᴺᵁᴸᴸ
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numeric_precision_radix: ᴺᵁᴸᴸ
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numeric_scale: ᴺᵁᴸᴸ
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datetime_precision: ᴺᵁᴸᴸ
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```
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[Original article](https://clickhouse.com/docs/en/operations/system-tables/columns) <!--hide-->
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