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doc-exports/docs/mrs/component-operation-guide-lts/mrs_01_24204.html
Yang, Tong 3f5759eed2 MRS comp-lts 2.0.38.SP20 version 
Reviewed-by: Hasko, Vladimir <vladimir.hasko@t-systems.com>
Co-authored-by: Yang, Tong <yangtong2@huawei.com>
Co-committed-by: Yang, Tong <yangtong2@huawei.com>
2023-01-19 17:08:45 +00:00

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ALTER TABLE: Modifying a Table Structure

This section describes the basic syntax and usage of the SQL statement for modifying a table structure in ClickHouse.

Basic Syntax

ALTER TABLE [database_name].name [ON CLUSTER cluster] ADD|DROP|CLEAR|COMMENT|MODIFY COLUMN ...

ALTER supports only *MergeTree, Merge, and Distributed engine tables.

Example

-- Add the test01 column to the t1 table.
ALTER TABLE t1 ADD COLUMN test01 String DEFAULT 'defaultvalue';
-- Query the modified table t1.
desc t1
┌─name────┬─type─┬─default_type─┬─default_expression ┬─comment─┬─codec_expression─┬─ttl_expression─┐
│  id          │ UInt8  │                │                     │           │                    │                  │  
│  name        │ String │                │                     │           │                    │                  │ 
│  address     │ String │                │                     │           │                    │                  │
│  test01      │ String │  DEFAULT       │  'defaultvalue'     │           │                    │                  │
└───────┴────┴────────┴────────── ┴───── ┴──────────┴─────────┘
-- Change the type of the name column in the t1 table to UInt8.
ALTER TABLE t1 MODIFY COLUMN name UInt8;
-- Query the modified table t1.
desc t1
┌─name────┬─type─┬─default_type─┬─default_expression ┬─comment─┬─codec_expression─┬─ttl_expression─┐
│  id          │ UInt8  │                │                     │           │                    │                  │  
│  name        │ UInt8  │                │                     │           │                    │                  │ 
│  address     │ String │                │                     │           │                    │                  │
│  test01      │ String │  DEFAULT       │  'defaultvalue'     │           │                    │                  │
└───────┴────┴────────┴────────── ┴───── ┴──────────┴─────────┘
-- Delete the test01 column from the t1 table.
ALTER TABLE t1 DROP COLUMN test01;
-- Query the modified table t1.
desc t1
┌─name────┬─type─┬─default_type─┬─default_expression ┬─comment─┬─codec_expression─┬─ttl_expression─┐
│  id          │ UInt8  │                │                     │           │                    │                  │  
│  name        │ UInt8  │                │                     │           │                    │                  │ 
│  address     │ String │                │                     │           │                    │                  │
└───────┴────┴────────┴────────── ┴───── ┴──────────┴─────────┘