ALTER COLUMN

Function

The ALTER TABLE ... ALTER COLUMN command is used to change the attributes of a column, such as the column type, position, and comment.

Syntax

ALTER TABLE Table name ALTER

[COLUMN] col_old_name TYPE column_type

[COMMENT] col_comment

[FIRST|AFTER] column_name

Parameter Description

**Table 1** ALTER COLUMN parameters
Parameter	Description
tableName	Table name.
col_old_name	Name of the column to be altered.
column_type	Type of the target column.
col_comment	Column comment.
column_name	New position to place the target column. For example, AFTER column_name indicates that the target column is placed after column_name.

Example

Changing the column type
```
ALTER TABLE table1 ALTER COLUMN a.b.c TYPE bigint
```
a.b.c indicates the full path of a nested column. For details about the nested column rules, see ADD COLUMNS.

The following changes on column types are supported:
- int => long/float/double/string/decimal
- long => float/double/string/decimal
- float => double/String/decimal
- From double to string or decimal
- From decimal to decimal or string
- From string to date or decimal
- From date to string

Altering other attributes

ALTER TABLE table1 ALTER COLUMN a.b.c DROP NOT NULL
ALTER TABLE table1 ALTER COLUMN a.b.c COMMENT 'new comment'
ALTER TABLE table1 ALTER COLUMN a.b.c FIRST
ALTER TABLE table1 ALTER COLUMN a.b.c AFTER x

a.b.c indicates the full path of a nested column. For details about the nested column rules, see ADD COLUMNS.