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# 系统。列 {#system-columns}

包含有关所有表中列的信息。

您可以使用此表获取类似于以下内容的信息 [DESCRIBE TABLE](../../sql-reference/statements/misc.md#misc-describe-table) 查询，但对于多个表一次。

该 `system.columns` 表包含以下列（列类型显示在括号中):

-   `database` (String) — Database name.
-   `table` (String) — Table name.
-   `name` (String) — Column name.
-   `type` (String) — Column type.
-   `default_kind` (String) — Expression type (`DEFAULT`, `MATERIALIZED`, `ALIAS`)为默认值，如果没有定义，则为空字符串。
-   `default_expression` (String) — Expression for the default value, or an empty string if it is not defined.
-   `data_compressed_bytes` (UInt64) — The size of compressed data, in bytes.
-   `data_uncompressed_bytes` (UInt64) — The size of decompressed data, in bytes.
-   `marks_bytes` (UInt64) — The size of marks, in bytes.
-   `comment` (String) — Comment on the column, or an empty string if it is not defined.
-   `is_in_partition_key` (UInt8) — Flag that indicates whether the column is in the partition expression.
-   `is_in_sorting_key` (UInt8) — Flag that indicates whether the column is in the sorting key expression.
-   `is_in_primary_key` (UInt8) — Flag that indicates whether the column is in the primary key expression.
-   `is_in_sampling_key` (UInt8) — Flag that indicates whether the column is in the sampling key expression.