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90 lines
4.8 KiB
Markdown
90 lines
4.8 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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Columns:
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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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**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: system
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table: aggregate_function_combinators
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name: name
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type: String
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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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Row 2:
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──────
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database: system
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table: aggregate_function_combinators
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name: is_internal
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type: UInt8
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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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```
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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) — Database name.
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- `table` (String) — Table name.
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- `name` (String) — Column name.
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- `type` (String) — Column type.
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- `default_kind` (String) — 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) — Expression for the default value, or an empty string if it is not defined.
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- `data_compressed_bytes` (UInt64) — The size of compressed data, in bytes.
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- `data_uncompressed_bytes` (UInt64) — The size of decompressed data, in bytes.
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- `marks_bytes` (UInt64) — The size of marks, in bytes.
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- `comment` (String) — Comment on the column, or an empty string if it is not defined.
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- `is_in_partition_key` (UInt8) — Flag that indicates whether the column is in the partition expression.
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- `is_in_sorting_key` (UInt8) — Flag that indicates whether the column is in the sorting key expression.
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- `is_in_primary_key` (UInt8) — Flag that indicates whether the column is in the primary key expression.
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- `is_in_sampling_key` (UInt8) — Flag that indicates whether the column is in the sampling key expression.
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[Original article](https://clickhouse.tech/docs/en/operations/system-tables/columns) <!--hide-->
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