# Evolve Iceberg table schema
<a name="querying-iceberg-evolving-table-schema"></a>

Iceberg schema updates are metadata-only changes. No data files are changed when you perform a schema update. 

The Iceberg format supports the following schema evolution changes:
+ **Add** – Adds a new column to a table or to a nested `struct`.
+ **Drop** – Removes an existing column from a table or nested `struct`.
+ **Rename** – Renames an existing column or field in a nested `struct`.
+ **Reorder** – Changes the order of columns.
+  **Type promotion** – Widens the type of a column, `struct` field, `map` key, `map` value, or `list` element. Currently, the following cases are supported for Iceberg tables: 
  + integer to big integer
  + float to double
  + increasing the precision of a decimal type

You can use the DDL statements in this section to modify Iceberg table schema.

**Topics**
+ [ALTER TABLE ADD COLUMNS](querying-iceberg-alter-table-add-columns.md)
+ [ALTER TABLE DROP COLUMN](querying-iceberg-alter-table-drop-column.md)
+ [ALTER TABLE CHANGE COLUMN](querying-iceberg-alter-table-change-column.md)
+ [SHOW COLUMNS](querying-iceberg-show-columns.md)

# ALTER TABLE ADD COLUMNS
<a name="querying-iceberg-alter-table-add-columns"></a>

Adds one or more columns to an existing Iceberg table.

## Synopsis
<a name="querying-iceberg-alter-table-add-columns-synopsis"></a>

```
ALTER TABLE [db_name.]table_name ADD COLUMNS (col_name data_type [,...])
```

## Examples
<a name="querying-iceberg-alter-table-add-columns-example"></a>

The following example adds a `comment` column of type `string` to an Iceberg table.

```
ALTER TABLE iceberg_table ADD COLUMNS (comment string)
```

The following example adds a `point` column of type `struct` to an Iceberg table.

```
ALTER TABLE iceberg_table 
ADD COLUMNS (point struct<x: double, y: double>)
```

The following example adds a `points` column that is an array of structs to an Iceberg table.

```
ALTER TABLE iceberg_table 
ADD COLUMNS (points array<struct<x: double, y: double>>)
```

# ALTER TABLE DROP COLUMN
<a name="querying-iceberg-alter-table-drop-column"></a>

Drops a column from an existing Iceberg table.

## Synopsis
<a name="querying-iceberg-alter-table-drop-column-synopsis"></a>

```
ALTER TABLE [db_name.]table_name DROP COLUMN col_name
```

## Example
<a name="querying-iceberg-alter-table-drop-column-example"></a>

```
ALTER TABLE iceberg_table DROP COLUMN userid
```

# ALTER TABLE CHANGE COLUMN
<a name="querying-iceberg-alter-table-change-column"></a>

Changes the name, type, order or comment of a column in an Iceberg table.

**Note**  
`ALTER TABLE REPLACE COLUMNS` is not supported. Because `REPLACE COLUMNS` removes all columns and then adds new ones, it is not supported for Iceberg. `CHANGE COLUMN` is the preferred syntax for schema evolution. 

## Synopsis
<a name="querying-iceberg-alter-table-change-column-synopsis"></a>

```
ALTER TABLE [db_name.]table_name
  CHANGE [COLUMN] col_old_name col_new_name column_type 
  [COMMENT col_comment] [FIRST|AFTER column_name]
```

## Example
<a name="querying-iceberg-alter-table-change-column-example"></a>

```
ALTER TABLE iceberg_table CHANGE comment blog_comment string AFTER id
```

# SHOW COLUMNS
<a name="querying-iceberg-show-columns"></a>

Shows the columns in a table.

## Synopsis
<a name="querying-iceberg-show-columns-synopsis"></a>

```
SHOW COLUMNS (FROM|IN) [db_name.]table_name
```

## Example
<a name="querying-iceberg-alter-table-change-column-example"></a>

```
SHOW COLUMNS FROM iceberg_table
```