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DESCRIBE TABLE

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Returns information about table columns.

Syntax

DESC|DESCRIBE TABLE [db.]table [INTO OUTFILE filename] [FORMAT format]

The DESCRIBE statement returns a row for each table column with the following String values:

  • name — A column name.
  • type — A column type.
  • default_type — A clause that is used in the column default expression: DEFAULT, MATERIALIZED or ALIAS. If there is no default expression, then empty string is returned.
  • default_expression — An expression specified after the DEFAULT clause.
  • comment — A column comment.
  • codec_expression — A codec that is applied to the column.
  • ttl_expression — A TTL expression.
  • is_subcolumn — A flag that equals 1 for internal subcolumns. It is included into the result only if subcolumn description is enabled by the describe_include_subcolumns setting.

All columns in Nested data structures are described separately. The name of each column is prefixed with a parent column name and a dot.

To show internal subcolumns of other data types, use the describe_include_subcolumns setting.

Example

Querysql
CREATE TABLE describe_example (
    id UInt64, text String DEFAULT 'unknown' CODEC(ZSTD),
    user Tuple (name String, age UInt8)
) ENGINE = MergeTree() ORDER BY id;

DESCRIBE TABLE describe_example;
DESCRIBE TABLE describe_example SETTINGS describe_include_subcolumns=1;
Responsetext
┌─name─┬─type──────────────────────────┬─default_type─┬─default_expression─┬─comment─┬─codec_expression─┬─ttl_expression─┐
│ id   │ UInt64                        │              │                    │         │                  │                │
│ text │ String                        │ DEFAULT      │ 'unknown'          │         │ ZSTD(1)          │                │
│ user │ Tuple(name String, age UInt8) │              │                    │         │                  │                │
└──────┴───────────────────────────────┴──────────────┴────────────────────┴─────────┴──────────────────┴────────────────┘

The second query additionally shows subcolumns:

Responsetext
┌─name──────┬─type──────────────────────────┬─default_type─┬─default_expression─┬─comment─┬─codec_expression─┬─ttl_expression─┬─is_subcolumn─┐
│ id        │ UInt64                        │              │                    │         │                  │                │            0 │
│ text      │ String                        │ DEFAULT      │ 'unknown'          │         │ ZSTD(1)          │                │            0 │
│ user      │ Tuple(name String, age UInt8) │              │                    │         │                  │                │            0 │
│ user.name │ String                        │              │                    │         │                  │                │            1 │
│ user.age  │ UInt8                         │              │                    │         │                  │                │            1 │
└───────────┴───────────────────────────────┴──────────────┴────────────────────┴─────────┴──────────────────┴────────────────┴──────────────┘

The DESCRIBE statement can also be used with subqueries or scalar expressions:

DESCRIBE SELECT 1 FORMAT TSV;

or

DESCRIBE (SELECT 1) FORMAT TSV;
Responsetext
1       UInt8

This usage returns metadata about the result columns of the specified query or subquery. It is useful for understanding the structure of complex queries before execution.

See Also

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