ALTER TABLE

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 ClickHouse cluster name] ADD|DROP|CLEAR|COMMENT|MODIFY COLUMN ...

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

Examples:

Create a table named DB_table1.

CREATE TABLE DB_table1 ON CLUSTER default_cluster(Year UInt16,Quarter UInt8,Month UInt8,DayofMonth UInt8,DayOfWeek UInt8,FlightDate Date,FlightNum String,Div5WheelsOff String,Div5TailNum String)ENGINE = MergeTree() PARTITION BY toYYYYMM(FlightDate) PRIMARY KEY (intHash32(FlightDate)) ORDER BY (intHash32(FlightDate),FlightNum) SAMPLE BY intHash32(FlightDate) SETTINGS index_granularity= 8192;

Add the test column to table DB_table1.

ALTER TABLE DB_table1 ADD COLUMN test String DEFAULT 'defaultvalue';

Query the table.

desc DB_tables;

Change the type of the Year column in the DB_table1 table to UInt8.
```
ALTER TABLE DB_table1 MODIFY COLUMN Year UInt8;
```
View the table structure.
```
desc DB_tables;
```
Delete the test column from the DB_table1 table.
```
ALTER TABLE DB_table1 DROP COLUMN test;
```
Query the table.
```
desc DB_tables;
```
Change the name of the Month column in the DB_table1 table to Month_test.
```
ALTER TABLE DB_table1 RENAME COLUMN Month to Month_test;
```
Query the table.
```
desc DB_tables;
```